As pointed by Marcin Slusarz <marcin.slusarz@gmail.com>, the warns happens
because CONFIG_DVB_MAX_ADAPTERS depends on CONFIG_DVB_CORE, and there are
valid configs where DVB_CORE is not selected.

This causes such warnings, for every V4L and common drivers that may or
may not be compiled with DVB support:

drivers/media/dvb/dvb-core/dvbdev.h:36:2: warning: #warning invalid CONFIG_DVB_MAX_ADAPTERS value

We can safely remove the warning.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>