ACS: Extract bw40/80/160 freqs out of acs_usable_bwXXX_chan()

ACS: Extract bw40/80/160 freqs out of acs_usable_bwXXX_chan()

This extracts the 3 lists of allowed channels for 40/80/160 MHz
bandwidth out of their respective functions. It also adds for each
segment the frequency of the segment's last channel and the index of the
segment's "center" channel.

This is preparative work to allow selecting a channel which is not the
first of its segment for 40/80/160 MHz. In addition, this adds the 5 GHz
160 MHz channel defined for 5735-5895 MHz (channels 149-177).

Signed-off-by: Nicolas Escande <nico.escande@gmail.com>