spi: SAM add spin lock around all tx/rx/txrx funcs

The fast tx/rx/txrx functions will leave the SPI bus in an
inoperable state if interrupted, potentially spinning forever waiting on
some data. Wrapping these operations in what amounts to a critical section
using spin locks to avoid the issue.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>