Add dma_retain() and dma_release() to reference count the users
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19344 a1c6a512-1295-4272-9138-f99709370657
* Adds a callback to be called on end of transfer
* Add a function to disable a channel
* Services the 2 channels if both are active in the isr
SD driver: panics on error
git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19333 a1c6a512-1295-4272-9138-f99709370657