libertas: make more functions static

These functions were used in the old debugfs code for events, but
as this code is now gone, there's no need to export those functions.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>