parisc: Use max() to calculate parisc_tlb_flush_threshold

Use max() to reduce 4 lines of code to a single line and improve its
readability.

Fixes the following Coccinelle/coccicheck warning reported by
minmax.cocci:

  WARNING opportunity for max()

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Helge Deller <deller@gmx.de>