Change usrp.selected_subdev() to return a weak reference to the

Change usrp.selected_subdev() to return a weak reference to the
daughterboard object.  This fixes the 'weakref exception' error on
shutdown that is sometimes seen.  What was happening is that user
code would hold a daughterboard reference, the USRP object would go
out of scope, and then when the daughterboard finally went out of scope,
it's destructor would try to invoke methods on (the now non-existent)
USRP.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@7442 221aa14e-8319-0410-a670-987f0aec2ac5