rtc: rx6110: fix build against modular I2C

With CONFIG_I2C=m, the #ifdef section is disabled, as shown
by this warning:

drivers/rtc/rtc-rx6110.c:314:12: error: unused function 'rx6110_probe' [-Werror,-Wunused-function]

Change the driver to use IS_ENABLED() instead, which works
for both module and built-in subsystems.

Fixes: afa819c2 ("rtc: rx6110: add i2c support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201230145938.3254459-1-arnd@kernel.org

drivers/rtc/rtc-rx6110.c

@@ -331,7 +331,7 @@ static int rx6110_probe(struct rx6110_data *rx6110, struct device *dev)
 	return 0;
 }
 
-#ifdef CONFIG_SPI_MASTER
+#if IS_ENABLED(CONFIG_SPI_MASTER)
 static struct regmap_config regmap_spi_config = {
 	.reg_bits = 8,
 	.val_bits = 8,
@@ -411,7 +411,7 @@ static void rx6110_spi_unregister(void)
 }
 #endif /* CONFIG_SPI_MASTER */
 
-#ifdef CONFIG_I2C
+#if IS_ENABLED(CONFIG_I2C)
 static struct regmap_config regmap_i2c_config = {
 	.reg_bits = 8,
 	.val_bits = 8,