USB: asus_oled.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: Jakub Schmidtke <sjakub@gmail.com>
CC: Pekka Paalanen <pq@iki.fi>
CC: Peter Huewe <peterhuewe@gmx.de>
CC: "Ken O'Brien" <kernel@kenobrien.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>