Fix panic and breakage for non-DMA ATA devices e.g. powermac macio cells.
Handle cases where dma function pointers may be NULL, and where
the max_iosize can't be derived from a DMA data structure. For
the latter, revert to the prior behaviour of using DFLTPHYS for
the max i/o size when there is no other data.

Reviewed by: marcel
No objection by: sos