net/bnx2x: fix DMAE timeout

In some cases, DPDK application may send packets
while PMD is going through load or unload flow.
This causes firmware to access invalid/unallocated
memory to process transmit buffer. Which results in
error on PCI bus and chip further blocks access to host,
causing a DMAE timeout.

Fix this issue by installing dummy empty transmit and receive
handlers at the beginning of unload path (rte_eth_dev_stop())
and install actual transmit and receive handlers after successful
load of the PMD port (rte_eth_dev_start()). This way, application
won't be able to send packets while device is going through
load/unload flow.

Fixes: 540a211084a7 ("bnx2x: driver core")
Cc: stable@dpdk.org
Signed-off-by: Shahed Shaikh <shshaikh@marvell.com>