net: fec: use dma_alloc_noncoherent for data cache enabled coldfire

Coldfire platforms with data caches can't properly implement
dma_alloc_coherent and currently just return noncoherent memory from
dma_alloc_coherent.

The fec driver than works around this with a flush of all caches in the
receive path. Make this hack a little less bad by using the explicit
dma_alloc_noncoherent API and documenting the hacky cache flushes so
that the DMA API level hack can be removed.

Also replace the check for CONFIG_M532x for said hack with a check
for COLDFIRE && !COLDFIRE_COHERENT_DMA.  While m532x is the only such
platform with a fec module, this makes the code more consistent and
easier to follow.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>