[media] v4l: s5p-tv: mixer: fix compilation warning

This patch fixes compilation warning in debug message.  The warning is caused
by incorrect 'unsigned' to 'unsigned long' conversion in dev_dbg.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>