mwifiex: fix a bug in Tx multiport aggregation

When aggregation port limit is reached, we stop aggregation and
the data is sent to firmware. It is observed that one less packet
than the port limit is aggregated in this case. ex. 15 instead of
16.
The reason is we have redundant port limit checks before current
packet is added to aggregation buffer.

The issue is fixed by removing these checks. We already have
necessary check in precopy current buffer handling.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>