mmc cleanups, mostly DMA/scatterlists

Minor MMC cleanup, mostly for DMA:

 - Tighten some parts of scatterlist handling:
     * switch to a NR_SG parameter (like other drivers), and use
       it to disable some code;
     * make max_seg_size reflect the EDMA limit (usually 2 MBytes)
       not the controller's multiblock request limit (64x bigger);
     * use standard scatterlist accessors.

 - Use more section annotations to shrink runtime driver footprint.

 - Fix whitespace glitches and remove an unused struct member, both
   left over from the "rip out 1/4..." patch.

 - Move some bits of code earlier, to simplify later patches.

 - Fix a few messages to terminate properly, with just a newline.

 - Report -EIO for DMA errors.

There should be at most very minor functional changes here.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>