ARC: bitops: Change __fls to return unsigned long

As per asm-generic definition and other architectures __fls should
return unsigned long.

No functional change is expected as return value should fit in unsigned
long.

Reviewed-by: Cezary Rojewski <cezary.rojewski@intel.com>
Signed-off-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>