drivers/i2c/designware/dw_i2c: return enum cb_err from dw_i2c_transfer

Using enum cb_err as return type instead of int improves the readability
of the code.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ic1812c4d8d2b4d9ad331a787bd302a4f0707c1fe
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61513
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

src/drivers/i2c/designware/dw_i2c.h

@@ -111,11 +111,8 @@ int dw_i2c_gen_speed_config(uintptr_t dw_i2c_addr,
 
 /*
  * Process given I2C segments in a single transfer
- * Return value:
- * -1 = failure
- *  0 = success
  */
-int dw_i2c_transfer(unsigned int bus,
+enum cb_err dw_i2c_transfer(unsigned int bus,
 			const struct i2c_msg *segments,
 			size_t count);