staging: wfx: fix Oops when CONFIG_OF_NET is not set

In most case, of_get_mac_address() return NULL in case of error.
However, if CONFIG_OF_NET is not set, it return -ENODEV.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-4-Jerome.Pouiller@silabs.com


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