mwifiex: display right transmit packet delay

drv_pkt_delay_max should be assigned non-zero value, so that
packet delay can be accumulate in the right way.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>