clocksource: exynos_mct: Fix bitmask regression for exynos4_mct_write

EXYNOS4_MCT_L_MASK is defined as 0xffffff00, so applying this bitmask
produces a number outside the range 0x00 to 0xff, which always results
in execution of the default switch statement.

Obviously this is wrong and git history shows that the bitmask inversion
was incorrectly set during a refactoring of the MCT code.

Fix this by putting the inversion at the correct position again.

Cc: stable@vger.kernel.org
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Reported-by: GP Orcullo <kinsamanka@gmail.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>