Fix Tango Read Only attribute

[JeanLucPons]

This PR fix wrong behavior for Tango read only attribute.
(This is a temporary fix that need better Ophyd investigation to be improved).

[TeresiaOlsson]

Looks good. The only thing I thought about is if:

        elif isinstance(self._cfg, (EpicsConfigW, EpicsConfigRW)):
            return self._cfg.write_pvname

should return a string with both the write and read PV. Or that won't work because of the expected return format?

[JeanLucPons]

name() is supposed to return the read pv name when read only and the write pv name otherwise while measure_name() should return the read pv name except for write only.