fix 40 MHz channel validation checks for the 2.4 GHz channel range

fix 40 MHz channel validation checks for the 2.4 GHz channel range

Just like the 5 GHz channels, 2.4 GHz channels are spaced 5 MHz apart.
40 MHz wide channels hence span channels [N, N + 4] not [N, N + 1].
Adjust our secondary channel range checks accordingly.