scsi: ufs: skip probing UFS card if it is not present

During bootup, we can avoid unnecessary delay of approximately 180ms during
UFS card initialization when the driver tries to communicate to an empty
UFS card slot.

Change-Id: Iab8e1d405fd3cdc10dbb28b1b1e0ab479c9fd880
Signed-off-by: Can Guo <cang@codeaurora.org>