Fix conditional on assertion for combining GPU Update and GPU coordinate send
Fixes a problem introduced in
9a3c2ce312c1e04634d636655388a8aaad53c8d1
which unintentionally blocks any use of GPU update with PME-PP
Communication because !stepWork.doNeighborSearch is missing from the
condition on the assertion.
Related to #3159 and #3160 which, when implemented, will remove the
restriction on coordinate send originating from the GPU (on non search steps).
Change-Id: Ie9b1102386b2c1bc927c09e4d8ead2bd2321320d