gpio: dwapb: clarify usage of the register file version

First of all, it's obvious that different versions can't be provided
simultaneously. Hence, versions can't be bit masks.

Second, due to above we have to mask out the version field in the flags
and only that can be evaluated against the certain version.

Clarify all above by:
 - introducing GPIO_REG_OFFSET_V1 and GPIO_REG_OFFSET_MASK
 - replacing conditional to mask out bits and compare to a version

Luckily there is no functional change, so no need to backport this.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Bartosz Golaszewski <brgl@bgdev.pl>