Revert "ARM: 7908/1: mm: Fix the arm_dma_limit calculation"

This reverts commit 787b0d5c since
it is no longer required after 7909/1 was applied, and it causes
build regressions when ARM_PATCH_PHYS_VIRT is disabled and DMA_ZONE
is enabled.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>