drivers: spi: fix the fast path on the SAM0 driver

To keep the bus fully loaded, the SAM0 has a fast path that recognises
special cases like TX only, RX only, or TX/RX of the same size.
Commit #ea2431f3 accidentally disabled this.

This increases the utilisation from around 30 % to around 90 % at 48
MHz.

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