asix: Fix tx transfer padding for full-speed USB

asix: Fix tx transfer padding for full-speed USB

[ Upstream commit 2a5809499e35b53a6044fd34e72b242688b7a862 ]

The asix.c USB Ethernet driver avoids ending a tx transfer with a zero-
length packet by appending a four-byte padding to transfers whose length
is a multiple of maxpacket. However, the hard-coded 512 byte maxpacket
length is valid for high-speed USB only; full-speed USB uses 64 byte
packets.

Signed-off-by: Ingo van Lil <inguin@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>