spi: qup: skip clk_disable_unprepare if the device is already runtime suspended

If the spi device is already runtime suspended, if spi_qup_suspend is
executed during suspend-to-idle or suspend-to-ram it will result in the
a splat from unpreparing a non-prepared clock.

This patch fixes the issue by executing clk_disable_unprepare conditionally
in spi_qup_suspend.

[Reworded commit message to remove irrelevant backtrace -- broonie]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>