staging: wfx: simplify API coherency check

The 'channel' argument of hif_join() should never be NULL. hif_join()
does not have the responsibility to recover bug of caller.

In current code, if the argument channel is NULL, memory leaks. The new
code just emit a warning and does not give the illusion that it is
supported (and indeed a Oops will probably raise a few lines below).

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20210913130203.1903622-13-Jerome.Pouiller@silabs.com


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