luci-mod-network: support newer wireless.wifi-device.band option

OpenWrt switched from "option hwmode" to "option band" in order to select
the frequency band to use for the radio phy.

Extend the channel selector to recognize and use an existing "option band"
to select the appropriate channel list. When operating upon a wireless
configuration still using "option hwmode", then translate it to a band
value internally and translate it back to "option hwmode" on save.

This should provide forward- and backwards compatibility with both current
OpenWrt master and older versions still using hwmode.

Fixes: #5106
Signed-off-by: Jo-Philipp Wich <jo@mein.io>