scsi: ufs: core: Capture command trace only for the cmd != NULL case

For the query request, we already have query_trace, but in
ufshcd_send_command(), there will add two more redundant traces. Since
lrbp->cmd is NULL in the query request, the two trace events below provide
nothing except the tag and DB. Instead of letting them take up the limited
trace ring buffer, it’s better not to print these traces in case of cmd ==
NULL.

ufshcd_command: send_req: ff3b0000.ufs: tag: 28, DB: 0x0, size: -1, IS: 0, LBA: 18446744073709551615, opcode: 0x0 (0x0), group_id: 0x0
ufshcd_command: dev_complete: ff3b0000.ufs: tag: 28, DB: 0x0, size: -1, IS: 0, LBA: 18446744073709551615, opcode: 0x0 (0x0), group_id: 0x0

Link: https://lore.kernel.org/r/20210531104308.391842-4-huobean@gmail.com
Reviewed-by: Can Guo <cang@codeaurora.org>
Signed-off-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>