staging: rtl8723bs: declare private function as static

This fixes a sparse warning regarding an undeclared symbol.  Since the
function is private to rtw_recv.c, it should be declared as static.

Signed-off-by: Kenneth Hsu <kennethhsu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>