mwifiex: reduce cloned skb queue size

Driver supports Tx status ack feature. When hostapd/
wpa_supplicant asks for ack status of an EAPOL/ACTION
frame, driver maintains a cloned skb for the packet
until TX_STATUS event is received from firmware.

Cloned skb queue gets flushed when connection is terminated
or driver is unloaded.

Let's reduce the queue size to avoid unnecessarily
keeping memory allocated when environment is busy.

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