igb: conditionalize I2C bit banging on external thermal sensor support

Commit a97f8783a937 ("igb: unbreak I2C bit-banging on i350") introduced
code to change I2C settings to bit banging unconditionally.

However, this patch introduced a regression:  On an Intel S2600CWR
Server Board with three NICs:

- 1x dual-port copper
  Intel I350 Gigabit Network Connection [8086:1521] (rev 01)
  fw 1.63, 0x80000dda

- 2x quad-port SFP+ with copper SFP Avago ABCU-5700RZ
  Intel I350 Gigabit Fiber Network Connection [8086:1522] (rev 01)
  fw 1.52.0

the SFP NICs no longer get link at all.  Reverting commit a97f8783a937
or switching to the Intel out-of-tree driver both fix the problem.

Per the igb out-of-tree driver, I2C bit banging on i350 depends on
support for an external thermal sensor (ETS).  However, commit
a97f8783a937 added bit banging unconditionally.  Additionally, the
out-of-tree driver always calls init_thermal_sensor_thresh on probe,
while our driver only calls init_thermal_sensor_thresh only in
igb_reset(), and only if an ETS is present, ignoring the internal
thermal sensor.  The affected SFPs don't provide an ETS.  Per Intel,
the behaviour is a result of i350 firmware requirements.

This patch fixes the problem by aligning the behaviour to the
out-of-tree driver:

- split igb_init_i2c() into two functions:
  - igb_init_i2c() only performs the basic I2C initialization.
  - igb_set_i2c_bb() makes sure that E1000_CTRL_I2C_ENA is set
    and enables bit-banging.

- igb_probe() only calls igb_set_i2c_bb() if an ETS is present.

- igb_probe() calls init_thermal_sensor_thresh() unconditionally.

- igb_reset() aligns its behaviour to igb_probe(), i. e., call
  igb_set_i2c_bb() if an ETS is present and call
  init_thermal_sensor_thresh() unconditionally.

Fixes: a97f8783a937 ("igb: unbreak I2C bit-banging on i350")
Tested-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Co-developed-by: Jamie Bainbridge <jbainbri@redhat.com>
Signed-off-by: Jamie Bainbridge <jbainbri@redhat.com>
Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://lore.kernel.org/r/20230214185549.1306522-1-anthony.l.nguyen@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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