brcm80211: cleanup function prototypes

- removed unneeded fn prototypes from include files.
- explicitly marked fn prototypes as extern in include files.
- reordered functions to account for removed forward declarations
  in include files.
- removed unused functions: brcms_c_txflowcontrol_override,
  brcms_c_txflowcontrol_prio_isset, brcms_c_txflowcontrol.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>