spi/omap100k: Convert to runtime PM

Currently the omap100k driver uses prepare and unprepare transfer hardware
to enable and disable clocks for the IP block. Since these functions are
called along with runtime PM and end up duplicating its functionality in a
less flexible fashion we are trying to phase them out so convert this
driver to do runtime PM instead.

While doing so add missing error handling and remove a redundant NULL
assignment.

Signed-off-by: Mark Brown <broonie@kernel.org>