staging: r8188eu: Make rtw_signal_stat_timer_hdl a static function

Make rtw_signal_stat_timer_hdl a static function instead of a global
function.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Chang Yu <marcus.yu.56@gmail.com>
Link: https://lore.kernel.org/r/20220623043855.407208-1-marcus.yu.56@gmail.com


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