stmmac: intel: Fix kernel crash due to wrong error path

Unfortunately sometimes ->probe() may fail. The commit b9663b7c
("net: stmmac: Enable SERDES power up/down sequence")
messed up with error handling and thus:

[   12.811311] ------------[ cut here ]------------
[   12.811993] kernel BUG at net/core/dev.c:9937!

Fix this by properly crafted error path.

Fixes: b9663b7c

 ("net: stmmac: Enable SERDES power up/down sequence")
Cc: Voon Weifeng <weifeng.voon@intel.com>
Cc: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>