staging: r8188eu: remove HW_VAR_MLME_JOIN

Remove the HW_VAR_MLME_JOIN case from SetHwReg8188EU() and move its
functionality to a new static function in rtw_mlme_ext.c. This is part
of the ongoing effort to get rid of the unwanted hal layer.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220409120627.10633-3-straube.linux@gmail.com


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