i2c: Test off by one in {piix4,vt596}_transaction()

With `while (timeout++ < MAX_TIMEOUT)' timeout reaches MAX_TIMEOUT + 1
after the loop. This is probably unlikely to produce a problem.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>