usbnet: add rx queue pausing

Add rx queue pausing to usbnet. This is needed by rndis_wlan so that it can
control rx queue and prevent received packets from being send forward before
rndis_wlan receives and handles 'media connect'-indication. Without this
establishing WPA connections is hard and fail often.

[v2] - removed unneeded use of skb_clone

Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>