i2c: core: Add stub for i2c_verify_client() if !CONFIG_I2C

If I2C is not compiled, there is no way we should see a call to
i2c_verify_client() on a device that is an i2c client. As such,
provide a stub to return NULL to resolve an associated build failure.

The build is failing with this link error
ld: fxls8962af-core.o: in function `fxls8962af_fifo_transfer':
fxls8962af-core.c: undefined reference to `i2c_verify_client'
Reported-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Fixes: af959b7b ("iio: accel: fxls8962af: fix errata bug E3 - I2C burst reads")
Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Acked-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20210603165835.3594557-1-jic23@kernel.org

include/linux/i2c.h

@@ -343,7 +343,6 @@ struct i2c_client {
 };
 #define to_i2c_client(d) container_of(d, struct i2c_client, dev)
 
-struct i2c_client *i2c_verify_client(struct device *dev);
 struct i2c_adapter *i2c_verify_adapter(struct device *dev);
 const struct i2c_device_id *i2c_match_id(const struct i2c_device_id *id,
 					 const struct i2c_client *client);
@@ -477,6 +476,13 @@ i2c_new_ancillary_device(struct i2c_client *client,
 			 u16 default_addr);
 
 void i2c_unregister_device(struct i2c_client *client);
+
+struct i2c_client *i2c_verify_client(struct device *dev);
+#else
+static inline struct i2c_client *i2c_verify_client(struct device *dev)
+{
+	return NULL;
+}
 #endif /* I2C */
 
 /* Mainboard arch_initcall() code should register all its I2C devices.