USB: usbip: convert to use DRIVER_ATTR_RW

We are trying to get rid of DRIVER_ATTR(), and the usbip driver
attribute can be trivially changed to use DRIVER_ATTR_RW().

Cc: Valentina Manea <valentina.manea.m@gmail.com>
Cc: <linux-usb@vger.kernel.org>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>