i2c: busses: i2c-cadence: Fix incorrectly documented 'enum cdns_i2c_slave_mode'

Fixes the following W=1 kernel build warning(s):

 drivers/i2c/busses/i2c-cadence.c:157: warning: expecting prototype for enum cdns_i2c_slave_mode. Prototype was for enum cdns_i2c_slave_state instead

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>