[PA-RISC] Fix time.c for new do_timer() calling convention

do_timer now wants to know how many ticks have elapsed.  Now that we
have to calculate that, we can eliminate some of the clever code that
avoided having to calculate that.  Also add some more documentation.
I'd like to thank Grant Grundler for helping me with this.

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>