net: asix: handle packets crossing URB boundaries

commit 8b5b6f5413e97c3e8bafcdd67553d508f4f698cd upstream.

ASIX AX88772B started to pack data even more tightly. Packets and the ASIX packet
header may now cross URB boundaries. To handle this we have to introduce
some state between individual calls to asix_rx_fixup().

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ Emil: backported to 3.2:
 - dropped changes to drivers/net/usb/ax88172a.c
 - Introduced some static function declarations as the functions
   are not used outside of asix.c (sparse is complaining about it) ]
Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>