scsi: qla2xxx: Zero undefined mailbox IN registers

While requesting a new mailbox command, driver does not write any data to
unused registers.  Initialize the unused register value to zero while
requesting a new mailbox command to prevent stale entry access by firmware.

Link: https://lore.kernel.org/r/20220713052045.10683-4-njavali@marvell.com
Cc: stable@vger.kernel.org
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bikash Hazarika <bhazarika@marvell.com>
Signed-off-by: Quinn Tran <qutran@marvell.com>
Signed-off-by: Nilesh Javali <njavali@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/qla2xxx/qla_mbx.c

@@ -238,6 +238,8 @@ qla2x00_mailbox_command(scsi_qla_host_t *vha, mbx_cmd_t *mcp)
 			ql_dbg(ql_dbg_mbx, vha, 0x1112,
 			    "mbox[%d]<-0x%04x\n", cnt, *iptr);
 			wrt_reg_word(optr, *iptr);
+		} else {
+			wrt_reg_word(optr, 0);
 		}
 
 		mboxes >>= 1;