usbnet: introduce usbnet 3 command helpers

This patch introduces the below 3 usb command helpers:

	usbnet_read_cmd / usbnet_write_cmd / usbnet_write_cmd_async

so that each low level driver doesn't need to implement them
by itself, and the dma buffer allocation for usb transfer and
runtime PM things can be handled just in one place.

Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>