scsi: sd: add null pointer check for scsi disk pointer

scsi disk driver probe first calls the PM runtime get before actually
allocating the scsi disk structure and if the PM runtime status was set
to suspended at this time then it will trigger scsi disk driver's
resume callback. But scsi disk driver's resume callback assumes that
scsi disk structure is always available when it is called but that's not
true always. Fix this issue by skipping the scsi disk resume if scsi disk
structure isn't allocated.

Change-Id: Ica920b1e07976140dc29847f4dc61aac9fa402ae
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>