mesh: Fix CONFIG_HT_OVERRIDES build without CONFIG_VHT_OVERRIDES

mesh: Fix CONFIG_HT_OVERRIDES build without CONFIG_VHT_OVERRIDES

Commit e5a9b1e8a3 ("mesh: Implement use of VHT20 config in mesh mode")
introduced the possibility to check the disable_vht param. However, this
entry is only available when CONFIG_VHT_OVERRIDES is enabled and as
such, this broke the build for some cases.

Fix this by encapsulating VHT property with the proper CONFIG entry.

Fixes: e5a9b1e8a3a5 ("mesh: Implement use of VHT20 config in mesh mode")
Signed-off-by: Arturo Buzarra <arturo.buzarra@digi.com>