gpio-f7188x: Fix concurrent GPIO accesses (and minor improvements)

[Vincent Pelletier <plr.vincent@xxxxxxxxx>]

Hello,

Making intensive use of the GPIOs on this SuperIO chip, I triggered a race
condition causing apparently the wrong GPIO pins to be modified (and likely
read) randomly.
For some reason (unclear to me, I'm not used at all to kernel API),
request_muxed_region/release_region as it is used in this driver is not
sufficient to serialize accesses.
Also, reading other drivers for the same chip, I see a different pattern:
rather than accessing GPIO registers through the common IO region, this
region is only used for device discovery. From there, per-function base
address is retrieved, and that IO region gets requested once.

So I fixed the concurrent access by:
- using the same access pattern as in other drivers
- adding mutexes around IO region accesses

Once this was done and I got a bit more comfortable with this driver, chip and
gpiolib, I continued and fixed 3 minor issues/lacks.

Regards,
--
Vincent Pelletier

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