TimerPort return values

[Mike Suchoff]

TimerPort::getTimer() and TimerPort::getElapsed() are documented as
return -1 when timer is inactive.
However these routines return a timeout_t which is defined as an unsigned
long.

I believe these routines should be documented as returning the value
TIMEOUT_INF when the timer is inactive.  That is, it returns the largest
unsigned 32 bit integer (all 1's).