[S390] dasd: get rid of compile warning

Get rid of this:

drivers/s390/block/dasd_eckd.c: In function 'dasd_eckd_build_cp_tpm_track.clone.8':
drivers/s390/block/dasd_eckd.c:2589:34: warning: 'len_to_track_end' may be used uninitialized in this function

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>