ARM: dma-mapping: fix build warning in __dma_alloc()

Fix build warning in __dma_alloc() as below:

arch/arm/mm/dma-mapping.c: In function '__dma_alloc':
arch/arm/mm/dma-mapping.c:653:29: warning: 'page' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>