arm/arm64: bitops: fix find_next_zero_bit to be compat with arm64

Current implementation of find_next_zero_bit() is incompatible with arm64.
Hence fix it by using BITS_PER_LONG define instead of constants and
use generic ffz() implementation.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>