tty: serial: samsung_tty: fix s3c24xx_serial_set_mctrl()

tty: serial: samsung_tty: fix s3c24xx_serial_set_mctrl()

S3C2410_UCON is a 32bit register, so it must be read with rd_regl()
instead of rd_reg(), otherwise the upper bits will be zeroed. Fix this.

Fixes: 72a43046b61a ("tty: serial: samsung_tty: loopback mode support")
Tested-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20220712140745.30362-1-m.szyprowski@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>