[SCSI] zfcp: Add comments to switch/case fallthroughs

Add comments where there is a deliberate fall through in switch/case
statements. This makes some code checkers happy and makes it clear
that there is no missing break statement.

Reviewed-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>