serial: samsung: Fix build for non-Exynos4210 devices

exynos4120_serial_drv_data is only defined when building with support
for Exynos4210 so use the already provided define to ensure that we
don't reference it when building for other SoCs.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
[kgene.kim@samsung.com: Fixed build warning]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>