usbnet: runtime wake up device before calling usbnet_{read|write}_cmd

This patch gets the runtime PM reference count before calling
usbnet_{read|write}_cmd, and puts it after completion of the
usbnet_{read|write}_cmd, so that the usb control message can always
be sent to one active device in the non-PM context.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>