Fix SPI dma problems; other cleanups

[dhalbert]

Helping to fix adafruit/circuitpython#10663, plus some other changes. Part of PR adafruit/circuitpython#10668

• shared_dma_transfer_* routines were passing the dma_descr_t struct by value, which caused the caller not to see changes made by the callee,. In particular .progressstruct member changes were lost. Changed to pass by pointer.
• Change name of that struct from dma_descr_t to dma_transfer_t to avoid confusion with hardware DMA descriptors also used in that code.
• Define and use NO_DMA_CHANNEL instead of AUDIO_DMA_CHANNEL_COUNT and DMA_CHANNEL_COUNT to indicate an invalid channel number.
• dma_allocate_channel(bool audio_channel) replaced by dma_allocate_audio_channel(void) and dma_allocate_non_audio_channel(void) to not pass control requests as arg and to clarify.
• Remove duplicated incorrect copy-pasta comments in samd/samd21/dma.c.
• dma.h: SAMD21 and SAMx5x have different numbers of DMA channels, so define DMA_CHANNEL_COUNT separately for each chip family.

Taggin @Randall-Scharpf for interest.

[tannewt]

Looks good to me. Thank you!