broadcom-wl: read channel using wlc on detect

Hardcoding channel number to 11 is a bad idea, it may be even not
available on some devices. If there are two single-band radios, the
one for 5GHz doesn't have 11 channel.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 36479