iwlwifi: return -ENOMEM instead of NULL when OOM in iwl_drv_start()

The callers of iwl_drv_start() are probe functions.  If a probe
function returns 0, it means it succeeded.  So if NULL was returned by
iwl_drv_start(), it would be considered as a success.

Fix this by returning -ENOMEM if the driver struct allocation fails in
iwl_drv_start().

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>