staging: wfx: fix missing 'static' keyword

Sparse tool noticed that wfx_enable_beacon() is never used outside of
sta.c. Therefore, it can be declared static.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200505123757.39506-11-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>