[SCSI] zfcp: fix compile warning

Get rid of this one:

drivers/s390/scsi/zfcp_erp.c: In function 'zfcp_erp_thread':
drivers/s390/scsi/zfcp_erp.c:1400: warning: ignoring return value of
'down_interruptible', declared with attribute warn_unused_result

zfcp_erp_thread is a kernel thread which can't receive any signals.
So introduce a dummy variable and get rid of the warning.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>