spi: spi-orion: check return value from clk_prepare()/clk_enable()

clk_prepare()/clk_enable() can fail, and it's return value should
be checked.  Add the proper checking, and while we're here, convert
to clk_prepare_enable().

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>