[iwm] if_iwm - Use chan list from ieee80211_scan_state for scan, not ic_channels.

• Limiting the channel list with "ifconfig wlan0 chanlist ..." now will actually set the list of channels scanned by iwm.

Tested:

• Intel 7260, STA mode, setting chanlist to 1-14 and 36-254, and indeed it does what it should!