wifi-scripts: fix vht_oper_centr_freq_seg0_idx_map calculation
When selecting channels above 100 in VHT160+ modes the center
frequency segment was incorrectly set to 50, causing the interface
to not come up.
Change logic to instead check if the channel is within ±28 channels
of the intended center, which matches the actual 160+ MHz channel
width specification for VHT160, HE160, and EHT160.
Signed-off-by: Sean Khan <datapronix@protonmail.com>
Link: https://github.com/openwrt/openwrt/pull/18013
Signed-off-by: John Crispin <john@phrozen.org>