mwifiex: minor corrections in multiport aggregation code

This patch includes below changes
1) Check if multiport aggregation feature flag while doing rx
length validation check.
2) Reset buffer size variables when buffers are freed.

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