ARM: S3C24XX: Fix Demux error in UART3 irqs on S3C2443 and S3C2416

IRQ_S3C2443_UART3 is being used as the base when it should actually
be IRQ_S3C2443_RX3 on S3C2443 and S3C2416 for the UART3.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>