USB: wacom: 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: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Ping Cheng <pingc@wacom.com>
Cc: Eduard Hasenleithner <eduard@hasenleithner.at>
Cc: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>