staging: brcm80211: Changed comments

Code cleanup. BCM4321 is not supported by this driver. Removed 'XXX' in
comments since they do not signal a critical issue to be patched. Removed
FIXME in comment since it signalled a problem in the Minstrel algorithm,
not in our driver.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>