i2c: Provide a stub for i2c_detect_slave_mode()

Drivers would like to call i2c_detect_slave_mode() even if !I2C_SLAVE.
Give them what they want to,

Otherwise kernel will not compile:
drivers/i2c/busses/i2c-designware-platdrv.c: In function ‘dw_i2c_plat_probe’:
drivers/i2c/busses/i2c-designware-platdrv.c:331:6: error: implicit declaration of function ‘i2c_detect_slave_mode’ [-Werror=implicit-function-declaration]
  if (i2c_detect_slave_mode(&pdev->dev))
      ^~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

Fixes: 6e38cf3b ("i2c: designware: Let slave adapter support be optional")
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

include/linux/i2c.h

@@ -295,6 +295,8 @@ static inline int i2c_slave_event(struct i2c_client *client,
 {
 	return client->slave_cb(client, event, val);
 }
+#else
+static inline bool i2c_detect_slave_mode(struct device *dev) { return false; }
 #endif
 
 /**