mhi: core: Fix find_last_bit() usage

find_last_bit() relies on unsigned long pointer arguments. Driver
uses a type cast that generates the KASAN warning. Replace
find_last_bit() with __fls() to pass the value and avoid casting
pointer to make the warning go away.

Change-Id: I247e5c425ba0c44d04b518bd6d4f351f9c700b29
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>