pps: use BUG_ON for kernel API safety checks

This way less overhead is involved when running production kernel.  If you
want to debug a pps client module please define DEBUG to enable the
checks.

Signed-off-by: Alexander Gordeev <lasaine@lvk.cs.msu.su>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>