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