msm8660-common: cifs: use offsetof macro

This cleans up the code by using the offsetof macro instead of
the &pSMB->hdr.Protocol construct. By implementing this, we avoid
array subscript compiler warnings.

Patch file referenced from:
https://git.samba.org/sfrench/?p=sfrench/cifs-2.6.git;a=commit;h=b2a3ad9ca502169fc4c11296fa20f56059c7c031

Change-Id: Ic70e975a760148923e53f4e5e0694d64195e38f5