[PARISC] Rewrite timer_interrupt() and gettimeoffset() using "unsigned" math.

It's just a bit easier to follow and timer code is complex enough.

So far, only tested on A500-5x (64-bit SMP), ie: gettimeoffset() code
hasn't been tested at all.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>