V4L/DVB (5884): zr36067: clean up debug function

Debugging cleanups to the zr36067 driver:

* Use module_param_named() to declare the debug parameter, so we can
  use a single global variable to handle the debug level. This makes
  the driver a bit smaller (by 648 bytes on x86_64), thanks to one
  less level of indirection on every use.

* Change the debug parameter sysfs permissions, so that the debug
  level can be adjusted at runtime, as is done in many other
  media/video drivers.

* The debug level is between 0 and 5, not 0 and 4.

* Move the zr_debug export and dprintk macro definition to a header
  file so that we don't have to define them in each source file.

* Simplify a duplicate test on zr_debug.

Note that zr_debug was subsequently renamed to debug_zr36067 to avoid
possible conflicts with other Zoran device drivers, on a suggestion
by Trent Piepho.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Acked-by: Ronald S. Bultje <rbultje@ronald.bitfreak.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>