[x86] Fix min_delta_ns for HPET clockchip

The core code does not use clockchip expire callback for immediate
timer events, instead it call set_next_event() with minimum possible
ticks.

The current value of min_delta_ns causes ramdom hits to BUG_ON() in
hpet set_next_event().

This patch fixes this issue by making hpet min_delta_ns equal to
100 microsecs.

Signed-off-by: Anup Patel <anup@brainfault.org>