net: wifi_utils: reduce `valid_5g_chans_20mhz` memory footprint

This change reduces the memory footprint by changing the data type of
`valid_5g_chans_20mhz` from `uint16_t` to `uint8_t`.

Additionally, since the maximum channel number for 5GHz Wi-Fi is 177,
it can fit within the `uint8_t` range.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>