scsi: ufs: replace debugfs mutex with spinlock

Using a mutex is problematic if the code can run in interrupt context.
In this patch we replace the ufs debugfs synchronization mechanism
from mutex to spinlock, which doesn't go to sleep.

Change-Id: I4e258898ed0ddf5b84d955fabd4354c2d2891c3d
Signed-off-by: Lee Susman <lsusman@codeaurora.org>
Signed-off-by: Yaniv Gardi <ygardi@codeaurora.org>