scsi: qedf: Fix crash during sg_reset

Driver was attempting to print cdb[0], which is not set for resets coming
from SCSI ioctls. Check for cmd_len before accessing cmnd.

Crash info:
[84790.864747] BUG: unable to handle kernel NULL pointer dereference at (null)
[84790.864783] IP: qedf_initiate_tmf+0x7a/0x6e0 [qedf]
[84790.865204] Call Trace:
[84790.865246]  scsi_try_target_reset+0x2b/0x90 [scsi_mod]
[84790.865266]  scsi_ioctl_reset+0x20f/0x2a0 [scsi_mod]
[84790.865284]  scsi_ioctl+0x131/0x3a0 [scsi_mod]

Signed-off-by: Arun Easi <aeasi@marvell.com>
Signed-off-by: Saurav Kashyap <skashyap@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>