[SCSI] zfcp: Reorder FCP I/O and task management handler functions

Instead of calling the same handler for both, I/O and task management
commands, use different handlers that call a function for the common
part.

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@suse.de>