if_cablemodem.h: Add parenthesis around ioctl macros

Protect the SIOCGCM* ioctl macros with parenthesis.

Reported-by: Paul Wouters <pwouters@redhat.com>
Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>