libertas: fix return from lbs_update_channel()

If we return the channel number in a 'ret' variable where anything
non-zero is later interpreted as an error, that isn't nice. It breaks
WPA, for a start. OLPC trac #5485

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>