USB: fix PTR_ERR translation in init_usb_class()

There is a misprint in init_usb_class():
IS_ERR is used to get error code instead of PTR_ERR.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>