asix: Continue processing URB if no RX netdev buffer

Avoid a loss of synchronisation of the Ethernet Data header 32-bit
word due to a failure to get a netdev socket buffer.

The ASIX RX handling algorithm returned 0 upon a failure to get
an allocation of a netdev socket buffer. This causes the URB
processing to stop which potentially causes a loss of synchronisation
with the Ethernet Data header 32-bit word. Therefore, subsequent
processing of URBs may be rejected due to a loss of synchronisation.
This may cause additional good Ethernet frames to be discarded
along with outputting of synchronisation error messages.

Implement a solution which checks whether a netdev socket buffer
has been allocated before trying to copy the Ethernet frame into
the netdev socket buffer. But continue to process the URB so that
synchronisation is maintained. Therefore, only a single Ethernet
frame is discarded when no netdev socket buffer is available.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Mark Craske <Mark_Craske@mentor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>