drivers: i2s_sam_ssc.c: store *dev_dma in flash

The DMA module the i2s_sam_ssc relies on cannot change during the
runtime. Store pointer to dev_dma in flash, not in RAM. The new
implementation saves 40 bytes of flash and 32 bytes of RAM.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>