scsi: ufs: Fix race condition in ufs qcom debugfs driver

In function ufs_qcom_dbg_testbus_cfg_write(), the global
variable ufs_qcom_host (host) is not protected by lock.
In function ufs_qcom_testbug_config(), we are checking this
variable in switch case and there is possibility of race
condition while accessing host variable in both of these
functions. This change fixes the possible race scenario
using spin_lock on host_lock.

Change-Id: I4e3fa1c3b80b92a648965371e12e52352cf80ce5
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>