[media] v4l2-common: add a debug macro to be used with dev_foo()

Currently, there's a mess at the V4L2 printk macros: some drivers
use their own macros, others use pr_foo() or v4l_foo() macros,
while more modern drivers use dev_foo() macros.

The best is to get rid of v4l_foo() macros, as they can be
replaced by either dev_foo() or pr_foo(). Yet, such change can
be disruptive, as dev_foo() cannot use KERN_CONT. So, the best
is to do such change driver by driver.

There are replacements for most v4l_foo() macros, but it lacks
a way to enable debug messages per level. So, add such macro,
in order to make the conversion easier.

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