spi: sam0: fix fast-rx path, and refresh comments

The SAM0 fast-path implementation was broken, and partially fixed in
commits 8181eede and 8a99bd0d...

This patch resolves an issue where the MSB is always zero on SAML21
parts, and appears to follow suit with the previous patches.

This patch also refreshes the commentary, and removes mention of the
"interleaved" operation that is no longer used - which appears to have
been problematic in the past.

In addition to this, it also resolves an off-by-one error in both the
fast_rx and fast_rxrx paths, which would have been tripped when
transmitting a zero-byte buffer.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>