tty: serial: Samsung: Fix return value

Return the value returned by the failing function instead
of -1 (which does not convey the right error information).

Fixes the following smatch warning:
drivers/tty/serial/samsung.c:1687 s3c24xx_serial_modinit() info:
why not propagate 'ret' from uart_register_driver() instead of -1?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>