spi: atmel: convert from legacy pm ops to dev_pm_ops

Instead of using legacy suspend/resume methods, using newer
dev_pm_ops structure allows better control over power management.
Also, duplicated 'return' is removed from atmel_spi_resume().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>