if_cablemodem.h: Add parenthesis around ioctl macros

[ Upstream commit 4f924b2aa4d3cb30f07e57d6b608838edcbc0d88 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>