pci: msm: Fix out of bound error with find_next_bit()

Existing api msm_pcie_write_reg_field() calls find_next_bit()
and results in a KASAN out of bounds error for taking in
the first parameter by typecasting as unsigned long type.
To avoid the KASAN error use __ffs() instead.

Change-Id: I9e155248e1233bbcb04ac5e2d1d6d512b3fa5387
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>