rtc: fix inverted resolution check

The current code in clock_register checks if the newly added clock has a
resolution value higher than the current one in order to make it the
default, which is wrong. Clocks with a lower resolution value should be
better than ones with a higher resolution value, in fact with the current
code FreeBSD is always selecting the worse clock.

Reviewed by: kib jhb jkim
Sponsored by: Citrix Systems R&D
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D6185