wl12xx: Assign value to rx msdu lifetime variable

The patch "wl12xx: cmd and acx interface rework" failed to assign MSDU
lifetime value in wl12xx_acx_rx_msdu_life_time() and breaks the functionality.
This patch fixes the regression by assigning the correct value.

Signed-off-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>