staging: brcm80211: remove extern function prototypes from c files

It is not recommended to specify function prototypes in c sources
using the extern keyword. Several occurrences have been removed or
placed in a header file or simply removed as it was already present
in a header file.

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