Get the current channel with 'iw dev wlanN link'

With older versions of iw we can not get the channel that we
are currently associated with. So we cannot detect if we have
to fallback to that channel. Because of this, hostapd exits
with the following errors:

    Failed to set beacon parameters
    Interface initialization failed
    ap0: interface state UNINITIALIZED->DISABLED
    ap0: AP-DISABLED
    ap0: Unable to setup interface.
    hostapd_free_hapd_data: Interface ap0 wasn't started

A solution is to get the frequency from 'iw dev wlanN link' and
convert it to the channel number.

Fix #50