ramips: limit 5GHz channels for RAVPower RP-WD009

ramips: limit 5GHz channels for RAVPower RP-WD009

When selecting a channel below 100 on the 5GHz radio, the channel will
be detected as busy all the time.

Survey data from wlan1
	frequency:			5240 MHz [in use]
	channel active time:		165729 ms
	channel busy time:		158704 ms
	channel transmit time:		0 ms

Channels 100 and above work fine:

Survey data from wlan1
	frequency:			5500 MHz
	channel active time:		133000 ms
	channel busy time:		21090 ms
	channel transmit time:		0 ms

Limit the available channels, so users do not have the impression
their device is broken.

Signed-off-by: David Bauer <mail@david-bauer.net>