net: txgbe: fix null pointer to pcs

For 1000BASE-X or SGMII interface mode, the PCS also need to be selected.
Only return null pointer when there is a copper NIC with external PHY.

Fixes: 02b2a6f9 ("net: txgbe: support copper NIC with external PHY")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Link: https://patch.msgid.link/20241115073508.1130046-1-jiawenwu@trustnetic.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>