staging: wfx: wfx_init_common() returns NULL on error

Smatch complains:

    bus_spi.c:228 wfx_spi_probe() warn: 'bus->core' could be an error pointer
    bus_sdio.c:221 wfx_sdio_probe() warn: 'bus->core' could be an error pointer

bus->core contains the result of wfx_init_common(). With this patch,
wfx_init_common() returns a valid pointer or NULL.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201009171307.864608-5-Jerome.Pouiller@silabs.com


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