Convert the x86 RTC driver to use new validated BCD<->timespec conversions.

New common routines were added to kern/subr_clock.c for converting between
calendrical time expressed in BCD and struct timespec. The new functions
return EINVAL on error, as expected when the clock hardware does not provide
valid time.

PR: 224813
Differential Revision: https://reviews.freebsd.org/D13731 (no reviewers)