net: netcp: ethss: fix errors in ethtool ops

In ethtool ops, it needs to retrieve the corresponding
ethss module (gbe or xgbe) from the net_device structure.
Prior to this patch, the retrieving procedure only
checks for the gbe module.  This patch fixes the issue
by checking the xgbe module if the net_device structure
does not correspond to the gbe module.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>