USB: wacom_sys.c: remove err() usage

err() was a very old USB-specific macro that I thought had
gone away.  This patch removes it from being used in the
driver and uses dev_err() instead.

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