net/enic: enable RQ first and then post Rx buffers

net/enic: enable RQ first and then post Rx buffers

Future VIC adapters may require that the driver enable RQ before
posting new buffers to the NIC. So split enic_alloc_rx_queue_mbufs()
into two functions, one that allocates buffers and fills RQ and the
other that posts them (i.e. PIO write to a doorbell). And, call the
post function only after enabling RQ.

Currently released models are not affected by this change, as they
work fine whether the driver posts buffers before or after enabling RQ.

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Reviewed-by: John Daley <johndale@cisco.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>