regmap-irq: Cleanup sizeof(...) use in memory allocation

Instead of mentioning unsigned int directly, use a sizeof(...)
involving the buffer we're allocating to ensure the types don't
get out of sync.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20220623211420.918875-4-aidanmacdonald.0x0@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>