arm64: Fix the typo in the API __dma_flush_area

Correct the API _dma_flush_area to __dma_flush_area
which is declared in this file.

Change-Id: I33d34071cd82055cb3ca5b0b502209077eb6a944
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>