spi: pxa2xx: check clk_prepare_enable() return value

clk_prepare_enable() can fail, so its return value should be checked and
acted upon.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: 3343b7a6

 ("spi/pxa2xx: convert to the common clk framework")
Signed-off-by: Tobias Jordan <Tobias.Jordan@elektrobit.com>
Signed-off-by: Mark Brown <broonie@kernel.org>