serial: s5p: Use livetree API to get "id" property

Use dev_read_u8_default() instead of fdtdec_get_int() to read the "id"
property from device tree, as suggested in [1]. dev_* API is already
used in this driver, so there is no reason to stick to fdtdec_* API.
This also fixes checkpatch warning:

    WARNING: Use the livetree API (dev_read_...)

[1] doc/develop/driver-model/livetree.rst

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>