staging: rtl8712: remove dead code

The while() loop will only exit in a return or a goto ask_for_joinbss,
which means it will never break and execute the return after it.
Removing return _FAIL since it is dead code.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>