drivers/timer: it51xxx: Compensate time calculation to reduce drift

The EC timer runs at 9.2 MHz, which leads to a non-integer number of
ticks per microsecond. This causes slight overestimation when delays
are computed using whole ticks (e.g., using 10 ticks for 1 µs).
To address this, the busy wait calculation is adjusted with a
compensation factor to minimize cumulative timing error over time.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>