net: ethernet: stmmac: fix write to sgmii_adapter_base

I made a mistake with the commit a6aaa003 ("net: ethernet: stmmac:
fix altr_tse_pcs function when using a fixed-link"). I should have
tested against both scenario of having a SGMII interface and one
without.

Without the SGMII PCS TSE adpater, the sgmii_adapter_base address is
NULL, thus a write to this address will fail.

Cc: stable@vger.kernel.org
Fixes: a6aaa003 ("net: ethernet: stmmac: fix altr_tse_pcs function when using a fixed-link")
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Link: https://lore.kernel.org/r/20220420152345.27415-1-dinguyen@kernel.org


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