mwifiex: fix a bug in Rx multiport aggregation logic

It's been observed Rx aggregated packets are always followed
by a single Rx packet. This patch improves our logic to add
that extra packet in next aggregation.

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