net: wifi_mgmt: linearise `scan_params->chan` arrays

Turn the nested arrays of `scan_params->chan` into a single array.
This adds the requirement to specify the band for each channel, but
eliminates the large amount of dead memory for unused bands. Overall,
this saves 50% of the RAM space for this variable.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>