dwc3-msm: Use __ffs() API instead of ffs() API

commit a247baee ("dwc3-msm: Add __iomem markings where needed")
added usage of ffs() API which results into wrong mask being used
while checking status of USB controller using DWC3_GDBGLTSSM register.
ffs() API always return 1-based bit position. Hence use __ffs() API
instead of ffs() API since we need 0-based bit position of the first
bit set to determine the shift amount.

Also replace other instances of find_first_bit() API by __ffs() API.

CRs-Fixed: 2075351
Change-Id: Ie4d81d52901e0270ca42834874c14a3b211590a6
Signed-off-by: Mayank Rana <mrana@codeaurora.org>