[SCSI] zfcp: Use correct width for timer_interval field

The timer_interval is 14 bits in width. Introduce a define for
properly masking the value.

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>