MIPS: partially inline dma ops

MIPS: partially inline dma ops

Several DMA ops are no-op on many platforms, and the indirection through
the mips_dma_map_ops function table is causing the compiler to emit
unnecessary code.

Inlining visibly improves network performance in my tests (on a 24Kc
based system), and also slightly reduces code size of a few drivers.

coherentio is changed from EXPORT_SYMBOL_GPL to EXPORT_SYMBOL, because
it is now potentially being accessed by any driver using the DMA mapping
API.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5913/