Obtain PV control values for the purpose of setting enum strings.

[hinxx]

I'm seeing self._data['enum_strs'] being set to None if I call enumStrings() after the enum PV received a value update. I got this explanation from pyepics pyepics/pyepics#276.

For me, the problem is solved by this PR, for example:

if self.isIoConnected() and self.isIoInitialized():
    self.pv_mode.enumStrings()