opr: fix often reported warning

opr: fix often reported warning

$ make CC=cgcc CHECK="smatch -Wsparse-all" 2>&1 | tee debug
$ sed -n 's/.*warning: //p' debug | sort | uniq -c | sort -n | tail -1
   7218 passing argument 1 of 'opr_AssertionFailed' discards 'const' qualifier from pointer target type [enabled by default]

Change-Id: I81c5c1db39b1a08c7dda3caa3d4cdf685186ff5b
Address: http://smatch.sourceforge.net/
Reviewed-on: http://gerrit.openafs.org/10019
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>