scsi: ufs: Check qcg before dereferencing

Check qcg before dereferencing to avoid NULL pointer
dereference and return if its NULL. This is is to
fix below kw warnings.

ufs-qcom.c:2489: 'qcg' is assigned the return value from
function 'cpu_to_group'.
ufs-qcom.c:2439: 'cpu_to_group' explicitly returns
a NULL value.
ufs-qcom.c:2490: 'qcg' is explicitly dereferenced.

Change-Id: I291c9230bb7fb5df414db8f80bfb17a50c8fc7cf
Signed-off-by: Nitin Rawat <nitirawa@codeaurora.org>