net: pch_gbe: remove mii_ethtool_gset() error handling

mii_ethtool_gset() does not return any errors, so error handling can be
omitted to make code more simple.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>