bnxt_en: Do not free port statistics buffer when device is down.

Port statistics which include RDMA counters are useful even when the
netdevice is down.  Do not free the port statistics DMA buffers
when the netdevice is down.  This is keep the snapshot of the port
statistics and counters will just continue counting when the
netdevice goes back up.

Split the bnxt_free_stats() function into 2 functions.  The port
statistics buffers will only be freed when the netdevice is
removed.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>