s390/dasd: remove unneeded code

Fix these set but not used warnings:

drivers/s390/block/dasd.c:3933:6: warning: variable 'rc' set but not used [-Wunused-but-set-variable]
drivers/s390/block/dasd_alias.c:757:6: warning: variable 'rc' set but not used [-Wunused-but-set-variable]

In addition to that remove the test if an unsigned is < 0:

drivers/s390/block/dasd_devmap.c:153:11: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>