net/s390 drivers: add missing 'const' attribute

Add missing 'const' attribute to avoid the following compile warnings:

drivers/s390/net/ctcm_main.c: In function 'ctcm_init':
drivers/s390/net/ctcm_main.c:1864: warning: assignment from incompatible pointer type
drivers/s390/net/lcs.c: In function 'lcs_init_module':
drivers/s390/net/lcs.c:2468: warning: assignment from incompatible pointer type
drivers/s390/net/claw.c: In function 'claw_init':
drivers/s390/net/claw.c:3408: warning: assignment from incompatible pointer type

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>