RDMA/cxgb3: Check return of kmalloc() in iwch_register_device()

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
[ Also remove cast from void * return of kmalloc() as suggested by  
  Jesper Juhl <jesper.juhl@gmail.com>. ]
Signed-off-by: Roland Dreier <rolandd@cisco.com>