hwclock: skip RTC_PARAM_SET for --param-set with unchanged value

hwclock: skip RTC_PARAM_SET for --param-set with unchanged value

Parameters set with `hwclock --param-set` tend to be persisted in the
RTC's EEPROM. Writing the same value over and over again can wear out
the EEPROM (e.g. on each boot).
So read the current value first. Only if the parameter is changed,
actually write the new value.

This allows for easier integrations, especially since there is no
machine-readable way of retrieving the current value via hwclock.

Signed-off-by: Bastian Krause <bst@pengutronix.de>