rtc: jz4740: Fix a typo in Kconfig

Fix typo introduced for RTC_DRV_JZ4740 in commit 586655d2 ("rtc:
jz4740: make the driver buildable as a module again").

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>