choose_devices: don't short-circuit dev choice when answers available

[matthiasdiener]

Otherwise, spurious errors can occur:

# only single device available on this platform:

$ clinfo -l
Platform #0: AMD Accelerated Parallel Processing
 `-- Device #0: gfx90a:sramecc+:xnack-  
$ PYOPENCL_CTX=AMD:0 python -c "import pyopencl; pyopencl.choose_devices()"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/p/lustre1/diener3/Work/enew/pyopencl/pyopencl/__init__.py", line 1674, in choose_devices
    raise RuntimeError("not all provided choices were used by "
pyopencl._cl.RuntimeError: not all provided choices were used by choose_devices. (left over: '0')

PYOPENCL_TEST=AMD:0 works fine in this context.

See also #781.

Please squash