b43: Fix rxheader channel parsing

This patch fixes the parsing of the RX data header channel field.

The current code parses the header incorrectly and passes a wrong
channel number and frequency for each frame to mac80211.
The FIXMEs added by this patch don't matter for now as the code
where they live won't get executed anyway. They will be fixed later.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>