mmc symbol cleanup

Get rid of some needless symbol duplication:

 - Use MMCST0_* symbols for both MMCST0 (irq status) and
   MMCIM (irq mask) registers.

 - Use MMCST0_* symbols instead of MMCSD_EVENT_* enums; the
   enum names obfuscated code with respect to chip specs.

 - Use mmc_resp_type() directly; RSP_TYPE() isn't needed.

Also:

 - Switch the MMCST0_* symbols to the more compact BIT() syntax,
   and add matching "what does this do?" comments.

 - Use the already-defined MMCCMD_* symbols, not magic numbers.

 - Remove obsolete lament about protocol layer not providing
   something which it now provides.

Minor functional changes that shouldn't affect behavior, and
were suggested by the above changes:

 - Don't enable read IRQs when writing data, and vice versa.

 - Shrink the PIO loop in the IRQ handler.

Overall, this code should now be easier to cross-check against
specs and thus more clear in several ways.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>