spi: sam0: fix txrx, NULL buffers, and add a stub async method.

This patches fixes a few bugs with the SAM0 driver:

- txrx was trasnmitting too many bytes
- adds support for NULL buffers to the fast paths
- fixes a NULL dereference on the rx buffer slow path

The tests under tests/driver/spi/spi_loopback now pass.

Signed-off-by: Michael Hope <mlhx@google.com>