net: ethernet: stmmac: dwmac-generic: fix probe error path

Make sure to call any exit() callback to undo the effect of init()
before returning on late probe errors.

Fixes: cf3f047b

 ("stmmac: move hw init in the probe (v2)")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>