vmxnet3: Fix RX descriptor length field overflow

RX descriptor length is 14 bit field, we overflow it
by allocating 16K buffers and device treats RX descriptor
buffer length as zero.

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