USB: appledisplay.c: remove dbg() usage

dbg() was a very old USB-specific macro that should no longer
be used. This patch removes it from being used in the driver
and uses dev_dbg() instead.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Mike Frysinger <vapier@gentoo.org>
CC: Arun Sharma <asharma@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>