staging: wilc1000: make function wilc_get_stats_async static

Makes local function wilc_get_stats_async() static

Cleans up sparse warning:
symbol 'wilc_get_stats_async' was not declared. Should it be static?

Signed-off-by: Marko Stankovic <dartnorris@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>