vmxnet3: Fix long RX packets processing

Function vmxnet3::rxq_eof() may exit in the middle
of multi-descriptor RX packet processing. In this case
it is expected to resume processing of the same packet
on next invocation.

Fixed by making corresponding pointers vmxnet3_rxqueue class
members instead of stack variables.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>