iio: core: Fix suspicious sizeof usage

Pointer size is variours in different system, say 32bit for 4 and 64bit
for 8. The 'sizeof(infomask)' may lead to wrong bit numbers.

Signed-off-by: Orson Zhai <orson.zhai@spreadtrum.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>