dm9000: Return an ERR_PTR() in all error conditions of dm9000_parse_dt()

In one error condition dm9000_parse_dt() returns NULL, however the
return value is checked using IS_ERR() in dm9000_probe(), leading to the
error not being properly propagated if CONFIG_OF is not enabled or the
device tree data is not available. Fix this by also returning an
ERR_PTR() in this case.

Fixes: 0b8bf1baabe5 (net: dm9000: Allow instantiation using device tree)
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>