[SCSI] zfcp: Update MAINTAINERS entry

Martin no longer works on zfcp, remove his entry from the MAINTAINERS
file and add Swen as co-maintainer.

Also remove the reference to the zfcpdump.txt file, zfcpdump is not
part of the zfcp device driver.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Acked-by: Martin Peschke <mpeschke@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>