[S390] monreader: fix dev_set_drvdata conversion

Fix this:

drivers/s390/char/monreader.c: In function 'mon_open':
drivers/s390/char/monreader.c:323: warning: passing argument 1 of 'dev_set_drvdata' from incompatible pointer type
include/linux/device.h:457: note: expected 'struct device *' but argument is of type 'struct device **'
drivers/s390/char/monreader.c: In function 'monreader_freeze':
drivers/s390/char/monreader.c:466: warning: passing argument 1 of 'dev_get_drvdata' from incompatible pointer type
include/linux/device.h:452: note: expected 'const struct device *' but argument is of type 'struct device **'

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