[SCSI] zfcp: Fix error handling for blocked unit for send FCP command

In the case the unit is blocked, zfcp_unit_get has not been called
yet, so the error handling path should not call zfcp_unit_put.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>