hostap: fix SAP UDP RX low throughput in 5GHz

The default Max A-MPDU length is 8191 shown in the VHT Cap Info from
SAP's beacon/Assoc response, and this length is read from
hapd->iface->conf->vht_capab. It will lead to the AMPDU only contains
5 packets and the throughput is only 60Mbps for 1x1 HE case in 5GHz.
Change the default vht_capab to VHT_CAP_MAX_A_MPDU_LENGTH_EXPONENT_MAX
(1048575), and the throughput can be increased to 83Mbps.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>