staging: wfx: relocate wfx_join() beside wfx_join_finalize()

wfx_join() and wfx_join_finalize() are the two halves of the association
process. Group them.

In addition, for better uniformity of the code, rename wfx_do_join() in
wfx_join().

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200907101521.66082-3-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>