[media] v4l2-subdev.h: fix sparse error with v4l2_subdev_notify

The notify function is a void function, yet the v4l2_subdev_notify
define uses it in a ? : construction, which causes sparse warnings.

Replace the define by a static inline function and move it to
v4l2-device.h, which is where it belongs since it needs to know the
v4l2_device struct. This wasn't a problem when it was a define, but
as a static inline function this no longer compiles in v4l2-subdev.h.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>