drivers/i2c/designware/dw_i2c: return enum cb_err from dw_i2c_init

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: I55e6d93ca141b687871ceaa763bbbbe966c4b4a3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61511
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jason Glenesk <jason.glenesk@gmail.com>

src/drivers/i2c/designware/dw_i2c.h

@@ -95,11 +95,8 @@ uintptr_t dw_i2c_base_address(unsigned int bus);
 
 /*
  * Initialize this bus controller and set the speed
- * Return value:
- * -1 = failure
- *  0 = success
 */
-int dw_i2c_init(unsigned int bus, const struct dw_i2c_bus_config *bcfg);
+enum cb_err dw_i2c_init(unsigned int bus, const struct dw_i2c_bus_config *bcfg);
 
 /*
  * Generate speed config based on clock