Make wwan_create_dev() to return either valid or error pointer,
In some cases it may return NULL. Prevent this by converting
it to the respective error pointer.

Fixes: 9a44c1cc ("net: Add a WWAN subsystem")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Link: https://lore.kernel.org/r/20210811124845.10955-1-andriy.shevchenko@linux.intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>