wifi-scripts: fix missing VHT capabilities detection

wifi-scripts: fix missing VHT capabilities detection

* Add missing parentheses in the conditionals for VHT160/VHT160-80PLUS80
  and VHT_MAX_MPDU capabilities. The missing parentheses caused the bitwise
  AND to be evaluated after the equality comparison due to ECMA's operator
  precedence, where `==` has higher precedence than `&`.

* Fix Max MPDU length detection by changing the comparison operators to
  `>=` vs `>` otherwise the condition would never be met.

* Add missing default values:
  - `true` value for `short_gi_80` (As it exists for `short_gi_20`, `short_gi_40`, `short_gi_160`)
  - `7` for `vht_max_mpdu` (Without it the loop in MAX-MPDU-* calculation always compares with null)

* Change the `vht160` condition to `config.vht160 <= 2`. This flag is
  `2` by default, and only ever set to `0` when `vht_oper_chwidth < 2`.

Signed-off-by: Sean Khan <datapronix@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/18013
Signed-off-by: John Crispin <john@phrozen.org>