Input: iqs7222 - preserve system status register

Input: iqs7222 - preserve system status register

Some register groups reserve a byte at the end of their continuous
address space. Depending on the variant of silicon, this field may
share the same memory space as the lower byte of the system status
register (0x10).

In these cases, caching the reserved byte and writing it later may
effectively reset the device depending on what happened in between
the read and write operations.

Solve this problem by avoiding any access to this last byte within
offending register groups. This method replaces a workaround which
attempted to write the reserved byte with up-to-date contents, but
left a small window in which updates by the device could have been
clobbered.

Now that the driver does not touch these reserved bytes, the order
in which the device's registers are written no longer matters, and
they can be written in their natural order. The new method is also
much more generic, and can be more easily extended to new variants
of silicon with different register maps.

As part of this change, the register read and write functions must
be gently updated to support byte access instead of word access.

Fixes: 2e70ef525b73 ("Input: iqs7222 - acknowledge reset before writing registers")
Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Link: https://lore.kernel.org/r/Z85Alw+d9EHKXx2e@nixie71
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>