ARM: fix warning caused by wrongly typed arm_dma_limit

arch/arm/mm/init.c: In function 'arm_memblock_init':
arch/arm/mm/init.c:380: warning: comparison of distinct pointer types lacks a cast

by fixing the typecast in its definition when DMA_ZONE is disabled.
This was missed in 4986e5c7

 (ARM: mm: fix type of the arm_dma_limit
global variable).

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