* avr i2c_master: Fix 1ms timeout
i2c_start() produces a minimum time_slice of 1ms for use as timeout
value.
The timer granularity is 1ms, it is entirely possible for timer_count
to tick up immediately after the last timer read and falsely trigger
timeout with a '>= 1' comparison.
* avr/drivers/i2c_master: Use timer_elapsed()
