Input: atmel_mxt_ts - only read messages in mxt_acquire_irq() when necessary

The workaround of reading all messages until an invalid is received is a
way of forcing the CHG line high, which means that when using
edge-triggered interrupts the interrupt can be acquired.

With level-triggered interrupts the workaround is unnecessary.

Also, most recent maXTouch chips have a feature called RETRIGEN which, when
enabled, reasserts the interrupt line every cycle if there are messages
waiting. This also makes the workaround unnecessary.

Note: the RETRIGEN feature is only in some firmware versions/chips, it's
not valid simply to enable the bit.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Acked-by: Benson Leung <bleung@chromium.org>
Acked-by: Yufeng Shen <miletus@chromium.org>
(cherry picked from ndyer/linux/for-upstream commit 1ae4e8281e491b22442cd5acdfca1862555f8ecb)
[gdavis: Fix conflicts due to v4.6-rc7 commit eb43335c4095 ("Input:
	 atmel_mxt_ts - use mxt_acquire_irq in mxt_soft_reset").]
Signed-off-by: George G. Davis <george_davis@mentor.com>
[jiada: reset use_retrigen_workaround at beginning of mxt_check_retrigen()
	call mxt_check_retrigen() after mxt_acquire_irq() in mxt_initialize()
	replace white-spaces with tab for MXT_COMMS_RETRIGEN
	Changed to check if IRQ is level type]
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Link: https://lore.kernel.org/r/20200727151637.23810-1-jiada_wang@mentor.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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