net: sfp: allow modules with slow diagnostics to probe

When a module is inserted, we attempt to read read the ID from address
0x50.  Once we are able to read the ID, we immediately attempt to
initialise the hwmon support by reading from address 0x51.  If this
fails, then we fall into error state, and assume that the module is
not usable.

Modules such as the ALCATELLUCENT 3FE46541AA use a real EEPROM for
I2C address 0x50, which responds immediately.  However, address 0x51
is an emulated, which only becomes available once the on-board firmware
has booted.  This prompts us to fall into the error state.

Since the module may be usable without diagnostics, arrange for the
hwmon probe independent of the rest of the SFP itself, retrying every
5s for up to about 60s for the monitoring to become available, and
print an error message if it doesn't become available.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

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