serial: mtk: initial priv data before using

This patch ensures driver private data being fully initialized in
_debug_uart_init which is not covered by .priv_auto ops.

Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/drivers/serial/serial_mtk.c b/drivers/serial/serial_mtk.c
index ded7346..2dffa14 100644
--- a/drivers/serial/serial_mtk.c
+++ b/drivers/serial/serial_mtk.c
@@ -439,6 +439,7 @@
 {
 	struct mtk_serial_priv priv;
 
+	memset(&priv, 0, sizeof(struct mtk_serial_priv));
 	priv.regs = (void *) CONFIG_VAL(DEBUG_UART_BASE);
 	priv.fixed_clk_rate = CONFIG_DEBUG_UART_CLOCK;