wifi-scripts: mac80211.sh: add EHT and HE160 support to iw_htmode

wifi-scripts: mac80211.sh: add EHT and HE160 support to iw_htmode

For WIFI7 devices (such as mt7925e), the dev width is currently
always "20 MHz (no HT)" in monitor mode.

Add EHT and HE160 support to iw_htmode to fix this issue.
Additionally, the following changes are made:
1. Set iw_htmode to 160MHz for VHT160. The reason for the current
   VHT160 setting is unclear and seems to have been in place for
   over a decade (ibss_htmode [1]). If anyone knows its impact,
   please inform me so I can restore it.
2. Modify MHZ to MHz. The original matching table in the current
   iw tool uses MHz. Although the match is case-insensitive,
   correcting this won't hurt.

[1]: https://github.com/openwrt/openwrt/commit/768d09be8781228d3553142c0f1e1231c6aacc7c

Signed-off-by: Ming Kuang <ming@imkuang.com>
Link: https://github.com/openwrt/openwrt/pull/18319
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>