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/test/dm/i2c.c b/test/dm/i2c.c
index ef88372..541b73b 100644
--- a/test/dm/i2c.c
+++ b/test/dm/i2c.c
@@ -67,10 +67,10 @@
 
 	ut_assertok(uclass_get_device_by_seq(UCLASS_I2C, busnum, &bus));
 	ut_assertok(i2c_get_chip(bus, chip, 1, &dev));
-	ut_assertok(i2c_set_bus_speed(bus, 100000));
+	ut_assertok(dm_i2c_set_bus_speed(bus, 100000));
 	ut_assertok(dm_i2c_read(dev, 0, buf, 5));
-	ut_assertok(i2c_set_bus_speed(bus, 400000));
-	ut_asserteq(400000, i2c_get_bus_speed(bus));
+	ut_assertok(dm_i2c_set_bus_speed(bus, 400000));
+	ut_asserteq(400000, dm_i2c_get_bus_speed(bus));
 	ut_assertok(dm_i2c_read(dev, 0, buf, 5));
 	ut_asserteq(-EINVAL, dm_i2c_write(dev, 0, buf, 5));