fix unnecessary switch to polling GPU wait

Commit 84597ae introduced an incorrect condition for testing
whether mdrun should and can switch to polling GPU wait.
This minor bug results in swithching to polling wait even when this
is unnecessary (i.e. when the driver in use is >=v5.0), This is
rather harmless, but the polling wait is fragile, so we should use
it only when really necessary.

This commit fixes the switching as well as reorganizes the conditionals
to make it more clear that switching is needed when ECC is on and the 
NVIDIA GPU driver is older than v5.0.

Change-Id: I9106cb34c9a8fde2f9b54045d9b03ee5f5d50d8b