Fix mis-use of LJ_CUTOFF_CHECK

This applies the same kind of fix as implemented in 8e974b0eb5bc in
gromacs/master, which was found and fixed after the SC fork.

If there's need to reproduce this fix on other branches, use something
like:

sed -i -e 's/LJ_CUTOFF_CHECK/VDW_CUTOFF_CHECK/g' \
    $(git ls-files src)
Merge pull request #79 from mabraham/opencl

Fix mis-use of LJ_CUTOFF_CHECK