dm: i2c: Add a dm_ prefix to driver model bus speed functions

As with i2c_read() and i2c_write(), add a dm_ prefix to the driver model
versions of these functions to avoid conflicts.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
diff --git a/common/cmd_i2c.c b/common/cmd_i2c.c
index 7c3ad00..fe8f77a 100644
--- a/common/cmd_i2c.c
+++ b/common/cmd_i2c.c
@@ -1730,7 +1730,7 @@
 #endif
 	if (argc == 1) {
 #ifdef CONFIG_DM_I2C
-		speed = i2c_get_bus_speed(bus);
+		speed = dm_i2c_get_bus_speed(bus);
 #else
 		speed = i2c_get_bus_speed();
 #endif
@@ -1740,7 +1740,7 @@
 		speed = simple_strtoul(argv[1], NULL, 10);
 		printf("Setting bus speed to %d Hz\n", speed);
 #ifdef CONFIG_DM_I2C
-		ret = i2c_set_bus_speed(bus, speed);
+		ret = dm_i2c_set_bus_speed(bus, speed);
 #else
 		ret = i2c_set_bus_speed(speed);
 #endif