spi/s3c64xx: Add missing pm_runtime_set_active() call in probe()

Mark device as PM runtime active during initialization to reflect
actual device power/clocks state. This reduces the enable count for SPI
bus controller gate clock so it can be disabled when the bus controller
is not used.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>