mwifiex: set valid tx_param during mwifiex_send_null_packet

While sending null packet from driver we are passing NULL
tx_param pointer to indicate there are no more packets in queue.
PCIe send routine assumes caller has done sanity check on
tx_param and may cause crash while dereferencing next_pkt_len
from tx_param.

Avoid this by passing tx_param structure with next_pkt_len as
zero instead of NULL pointer.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>