serial: samsung: Fix possible out of bounds access on non-DT platform

On non-DeviceTree platforms, the index of serial device is a static
variable incremented on each probe.  It is incremented even if deferred
probe happens when getting the clock in s3c24xx_serial_init_port().

This index is used for referencing elements of statically allocated
s3c24xx_serial_ports array.  In case of re-probe, the index will point
outside of this array leading to memory corruption.

Increment the index only on successful probe.

Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Fixes: b497549a ("[ARM] S3C24XX: Split serial driver into core and per-cpu drivers")
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>