MIPS: improve checks for noncoherent DMA

Only one MIPS development board actually supports enabling/disabling DMA
coherency at runtime, so it's not a good idea to push the overhead of
checking that configuration setting onto every other supported target as
well.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5912/