Remove some significant duplication:

 - We only need a single bitmap to record which PARAM slots are
   in use, and it only needs to have enough bits to cover the
   slots that exist.

 - By using the atomic bitops for that, we eliminate the current
   need for a private spinlock.

 - We don't need to record 'tcc' either, it's only needed inside
   davinci_request_dma().  Remove it:  be clearer and more correct
   (it can change with PARAM operations), save space.

This change highlighted some existing bugs in terms of fault returns
when davinci_request_dma() couldn't return a resource of the relevant
type; unlikely for anyone to have hit them, so far.

Switch to standard kerneldoc for davinci_request_dma().  The two
previous descriptive comments were inconsistent, so fix that too.
Explain the callback usage a bit.

Minor new feature:  allow explicit allocation of slave channels too,
allowing pre-allocation of *any* DMA channel on behalf of DSP code.

This saves about 2 KB of space (half is data) as well as making
allocation and deallocation code a LOT simpler.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>