p54: improve site survey

p54: improve site survey

The firmware keeps track of channel usage. This data can
be used by the automatic channel selection to find the best
channel.

Survey data from wlan4
	frequency:				5200 MHz [in use]
	noise:					-91 dBm
	channel active time:			811909 ms
	channel busy time:			63395 ms
	channel transmit time:			59636 ms
Survey data from wlan4
	frequency:				5210 MHz
	noise:					-91 dBm
	channel active time:			121 ms
	channel busy time:			119 ms
	channel transmit time:			0 ms

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>