niu: Fix error handling

platform_device_register_simple() returns ERR_PTR(), not NULL, if an error 
occurs.

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

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>