iwlegacy: 3945: fix hw passive scan on radar channels

iwlegacy: 3945: fix hw passive scan on radar channels

Patch fix firmware error on "iw dev wlan0 scan passive" for
hardware scanning (with disable_hw_scan=0 module parameter).

 iwl3945 0000:03:00.0: Microcode SW error detected. Restarting 0x82000008.
 iwl3945 0000:03:00.0: Loaded firmware version: 15.32.2.9
 iwl3945 0000:03:00.0: Start IWL Error Log Dump:
 iwl3945 0000:03:00.0: Status: 0x0002A2E4, count: 1
 iwl3945 0000:03:00.0: Desc       Time       asrtPC blink2 ilink1  nmiPC   Line
 iwl3945 0000:03:00.0: SYSASSERT     (0x5) 0041263900 0x13756 0x0031C 0x00000 764
 iwl3945 0000:03:00.0: Error Reply type 0x000002FC cmd C_SCAN (0x80) seq 0x443E ser 0x00340000
 iwl3945 0000:03:00.0: Command C_SCAN failed: FW Error
 iwl3945 0000:03:00.0: Can't stop Rx DMA.

We have disable ability to change passive scanning to active on
particular channel when traffic is detected on that channel. Otherwise
firmware will report error, when we try to do passive scan on radar
channels.

Reported-and-debugged-by: Pedro Francisco <pedrogfrancisco@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>