scsi: ufs: Use enum dev_cmd_type where appropriate

Declare all variables that hold dev_cmd_type values as an enum instead of
as an int.

Cc: Yaniv Gardi <ygardi@codeaurora.org>
Cc: Subhash Jadavani <subhashj@codeaurora.org>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Avri Altman <avri.altman@wdc.com>
Cc: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20191029230710.211926-3-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>