rtc: ds1307: update reference to removed CONFIG_RTC_DRV_DS1307_HWMON

rtc: ds1307: update reference to removed CONFIG_RTC_DRV_DS1307_HWMON

The CONFIG_RTC_DRV_DS1307_HWMON macro was removed in favor of
CONFIG_HWMON in commit 6b583a64fd1e ("rtc: ds1307: simplify hwmon
config"), but a reference to it remained in a comment. Correct this
reference.

Discovered while searching for CONFIG_* symbols referenced in code but
not defined in any Kconfig file.

Signed-off-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
Link: https://patch.msgid.link/20260610054723.261008-1-enelsonmoore@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>