drivers: spi: sam/sam0: Don't claim support for async transfers

Asyc transceive is currently not implemented for this driver,
so don't claim support.

spi_context_lock() is already being called in spi_sam_transceive()
so calling it in the wrapper function will cause a deadlock.

This reverts eae05d92

Signed-off-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>