staging: wfx: take advantage of IS_ERR_OR_NULL()

Obviously, current code can be replaced by IS_ERR_OR_NULL().

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-16-Jerome.Pouiller@silabs.com


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