gnss: fix potential error pointer dereference

The gnss_allocate_device() function returns a mix of NULL and error
pointers on error.  It should only return one or the other.  Since the
callers both check for NULL, I've modified it to return NULL on error.

Fixes: 2b6a4403 ("gnss: add GNSS receiver subsystem")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>