AP: Add initial support for 6 GHz band

AP: Add initial support for 6 GHz band

Add support for new hardware mode for 6 GHz band. 6 GHz operation is
defined in IEEE P802.11ax/D4.3. 6 GHz band adds global operating classes
131-135 that define channels in frequency range from 5940 MHz to 7105
MHz.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
- Remove HOSTAPD_MODE_IEEE80211AX mode
- Replace check for HOSTAPD_MODE_IEEE80211AX with is_6ghz_freq()
- Move center_idx_to_bw_6ghz() to ieee802_11_common.c file

Signed-off-by: Vamsi Krishna <vamsin@codeaurora.org>