clk: shmobile: mstp: Fix the is_enabled() operation

The MSTP[SC]R registers have clock stop bits, not clock enable bits. The
bit value should thus be inverted in the is_enabled() operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mike Turquette <mturquette@linaro.org>