regmap-irq: Minor adjustments to .handle_mask_sync()

If a .handle_mask_sync() callback is provided it supersedes all
inbuilt handling of mask registers, and judging by the commit
69af4bca ("regmap-irq: Add handle_mask_sync() callback") it
is intended to completely replace all default IRQ masking logic.

The implementation has two minor inconsistencies, which can be
fixed without breaking compatibility:

(1) mask_base must be set to enable .handle_mask_sync(), even
    though mask_base is otherwise unused. This is easily fixed
    because mask_base is already optional.

(2) Unmask registers aren't accounted for -- they are part of
    the default IRQ masking logic and are just a bit-inverted
    version of mask registers. It would be a bad idea to allow
    them to be used at the same time as .handle_mask_sync(),
    as the result would be confusing and unmaintainable, so
    make sure this can't happen.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20230511091342.26604-4-aidanmacdonald.0x0@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>