mwifiex: fix 5G association failure after leaving 2.4G IBSS

When assocaiting to an AP , mwifiex set supported data rates
based on target AP's capability. For 5G AP(11a mode), a/n/ac mode
would possibly be set.
However, for some chips which dont support 11AC mode current config_bands
will be used instead.

For example, if we join an IBSS in 11b mode ,adapter->config_bands
will be set to 1(11b mode). Then we leave IBSS ,and try to connect
5G a/n mode AP. At this time , only 11b mode data rates will be
supported in assoc request , which result in assoc failure with
reason code 18: Association denied due to requesting station not
supporting all rates.

This patch fix such a cornel case, by adding additional check for
current chip's 11ac capability.

Reported-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>