[media] saa7134.h: Suppress compiler warnings when CONFIG_VIDEO_SAA7134_RC is not set

If the said config optio is not set, the compiler will spill out many
warnings about statements with no effect, such as:

Casting the zero to void will cure the warning.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>