[S390] dasd: fix endless loop in erp

If not enough memory is available to build a new erp request it ended
up in an endless loop trying to build erp requests. Fixed the loop to
proceed the next request instead.

Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>