staging:r8192u: replace request_module with try_then_request_module

Return value of request_module() does not handled,
so it is possible to use shorter try_then_request_module().

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>