media: dvbdev: handle ENOMEM error at dvb_module_probe()

If allocation of struct board_info fails, return NULL from
dvb_module_probe().

Fix this warning:
	drivers/media/dvb-core/dvbdev.c:958 dvb_module_probe() error: potential null dereference 'board_info'.  (kzalloc returns null)

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>