scsi:ufs: make probe as async probe

Ufs probe use sleep and takes time to finish,
which stalls kernel booting.
So, make ufs probe as asynchronously to improve
the booting time.

Change-Id: Icaf0bd33476a1949a1338a30f4dc2ade52861350
Signed-off-by: Lei wang <leiwan@codeaurora.org>