x86/time: minor adjustments to init_pit()

x86/time: minor adjustments to init_pit()

For one, "using_pit" shouldn't be set ahead of the function's last
(for now: only) error path. Otherwise "clocksource=pit" on the command
line can lead to misbehavior when actually taking that error path.

And then make an implicit assumption explicit: CALIBRATE_FRAC cannot,
for example, simply be changed to 10. The way init_pit() works, the
upper bound on the calibration period is about 54ms.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>