memory: samsung: exynos5422-dmc: Reduce protected code area in IRQ handler

Speed-up a bit this IRQ processing as there is no need to protect
return value or printing.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Acked-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>