hw/exynos4210_combiner.c: Drop excessive read/write access check.

Access to reserved area at offset higher than 0x3c is allowed in
External Combiner. Samsung Galaxy Kernel implements this. So, drop
excessive checks in read/write functions.

Signed-off-by: Evgeny Voevodin <e.voevodin@samsung.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
