Save a memory dereference in the ISR by passing this in directly.
Calling pps_capture is MP safe for all other operations on struct
pps_state, so there's no need to aquire the lock before we do this,
even from a fast ISR. Avoid dereferencing sc->ppbus until after
pps_capture is called as well. These actions reduce somewhat the
cache effects that cause variance in interrupt times. On an
especially slow test machine (300MHz Cyrix GXm), this reduces the
interrupt latency about about 10% (from 21us to 19us) and helps a
little with the variance (although most of the variance seems to be
caused by lots of interrupt masking).
This also happens fixes one or two of bde's style issues.
7f3e0c4d13