imx-drm: imx-ldb: Use snprintf()

Use snprintf() in order to fix the following static checker warning:

drivers/staging/imx-drm/imx-ldb.c:340 imx_ldb_get_clk() error: format string overflow. buf_size: 16 length: 18
probably 18 is theory and not real life, but 16 is based on
theory as well.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>