i2c: Provide a stub for i2c_detect_slave_mode()

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: 6e38cf3b4421 ("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>