Revert "Mask range.unit with 0xff when determining the unit."

Revert "Mask range.unit with 0xff when determining the unit."

This reverts commit d721a0c05450c793a5e0e562d286a689565cf81b.

From the National Instruments E Series User Manual [1], page 3-2:

  Reference Selection

  (NI 6020E, NI PXI-6040E, NI 6052E, NI 6062E, NI 6070E/6071E, and
  PCI-MIO-16E-4 Devices Only) You can connect each DAC to the device
  internal reference of 10 V or to the external reference signal
  connected to the external reference (AO EXT REF) pin on the I/O
  connector. This signal applied to EXT REF should be within
  ±11 V. You do not need to configure both channels for the same mode.

So RF_EXTERNAL is a valid unit (i.e. in units of the AO EXT REF
signal).  The next commit will add this unit.

[1]: http://digital.ni.com/manuals.nsf/websearch/06115536722DAA558625728E007E6BA4
  Edition Date:					February 2007
  Part Number:   					370503K-01
  p