USB: at76c50x-usb.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: "John W. Linville" <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>