hppa: simplify itmr_rearm()

hppa: simplify itmr_rearm()

The nest of branches in itmr_rearm() can be simplified to:

	if (cycles <= t1 - t0) {
		/* we probably missed */
	}

We're doing modular unsigned 32-bit, so the rollover case in the
current code is superfluous.

Tested by miod@.

"Works for me." miod@