i2c: gxp: fix build failure without CONFIG_I2C_SLAVE

The gxp_i2c_slave_irq_handler() is hidden in an #ifdef, but the
caller uses an IS_ENABLED() check:

drivers/i2c/busses/i2c-gxp.c: In function 'gxp_i2c_irq_handler':
drivers/i2c/busses/i2c-gxp.c:467:29: error: implicit declaration of function 'gxp_i2c_slave_irq_handler'; did you mean 'gxp_i2c_irq_handler'? [-Werror=implicit-function-declaration]

It has to consistently use one method or the other to avoid warnings,
so move to IS_ENABLED() here for readability and build coverage, and
move the #ifdef in linux/i2c.h to allow building it as dead code.

Fixes: 4a55ed6f ("i2c: Add GXP SoC I2C Controller")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nick Hawkins <nick.hawkins@hpe.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-gxp.c

@@ -353,7 +353,6 @@ static void gxp_i2c_chk_data_ack(struct gxp_i2c_drvdata *drvdata)
 	writew(value, drvdata->base + GXP_I2CMCMD);
 }
 
-#if IS_ENABLED(CONFIG_I2C_SLAVE)
 static bool gxp_i2c_slave_irq_handler(struct gxp_i2c_drvdata *drvdata)
 {
 	u8 value;
@@ -437,7 +436,6 @@ static bool gxp_i2c_slave_irq_handler(struct gxp_i2c_drvdata *drvdata)
 
 	return true;
 }
-#endif
 
 static irqreturn_t gxp_i2c_irq_handler(int irq, void *_drvdata)
 {

include/linux/i2c.h

@@ -385,7 +385,6 @@ static inline void i2c_set_clientdata(struct i2c_client *client, void *data)
 
 /* I2C slave support */
 
-#if IS_ENABLED(CONFIG_I2C_SLAVE)
 enum i2c_slave_event {
 	I2C_SLAVE_READ_REQUESTED,
 	I2C_SLAVE_WRITE_REQUESTED,
@@ -396,9 +395,10 @@ enum i2c_slave_event {
 
 int i2c_slave_register(struct i2c_client *client, i2c_slave_cb_t slave_cb);
 int i2c_slave_unregister(struct i2c_client *client);
-bool i2c_detect_slave_mode(struct device *dev);
 int i2c_slave_event(struct i2c_client *client,
 		    enum i2c_slave_event event, u8 *val);
+#if IS_ENABLED(CONFIG_I2C_SLAVE)
+bool i2c_detect_slave_mode(struct device *dev);
 #else
 static inline bool i2c_detect_slave_mode(struct device *dev) { return false; }
 #endif