[PARISC] Fix parisc_setup_cache_timing to choose a better flush threshold

update comment about CAFL_STRIDE

Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
Fixed a bug in parisc_setup_cache_timing() which caused it to calculate
a poor value for parisc_cache_flush_threshold.

Thanks to Joel Soete for spotting the bug.
Thanks to James Bottomley for pointing out the clean way to fix this.

Signed-off-by: Stuart Brady <sdb@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>