wireless: ERR_PTR vs null

iwm_wdev_alloc() returns an ERR_PTR on failure and not null.  It also
prints its own dev_err() message so I removed that as well.

Compile tested only.  Sorry.
Found by smatch (http://repo.or.cz/w/smatch.git

).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>