Impact: bug fix

Currently if you do (e.g.) timec -e -1 ls, it will report 0 for the
value of the cpu_clock counter.  The reason is that the core assumes
that a counter's count field is up-to-date when the counter is inactive,
and doesn't call the counter's read function.  However, the cpu_clock
counter code only updates the count in the read function.

This fixes it by making both the read and disable functions update the
count.  It also makes the counter ignore time passing while the counter
is disabled, by making the enable function update the hw.prev_count field.
Signed-off-by: Paul Mackerras <paulus@samba.org>