[media] saa7146: remove return after BUG()

As reported by smatch:
	drivers/media/common/saa7146/saa7146_fops.c:314 fops_mmap() info: ignoring unreachable code.
	drivers/media/common/saa7146/saa7146_fops.c:402 fops_read() info: ignoring unreachable code.
	drivers/media/common/saa7146/saa7146_fops.c:426 fops_write() info: ignoring unreachable code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>