spi: Do not require a completion

There is no real reason why we require transfers to have a completion and
the only user of the completion now checks to see if one has been provided
before using it so stop enforcing this. This makes it more convenient for
drivers to chain multiple asynchronous transfers together.

Signed-off-by: Mark Brown <broonie@linaro.org>
(cherry picked from commit 1e25cd47)
(cherry picked from commit 3d780e874e52daf0a976460a62f520cb438b6743)