net: tsec: Mark tsec_get_interface as __maybe_unused

Non DM builds fail with the following error:
drivers/net/tsec.c:641:24: error: 'tsec_get_interface' defined but not used [-Werror=unused-function]
  641 | static phy_interface_t tsec_get_interface(struct tsec_private *priv)

Fix that.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/drivers/net/tsec.c b/drivers/net/tsec.c
index ee820aa..260ae88 100644
--- a/drivers/net/tsec.c
+++ b/drivers/net/tsec.c
@@ -638,7 +638,7 @@
 	return priv->phydev->link ? 0 : -1;
 }
 
-static phy_interface_t tsec_get_interface(struct tsec_private *priv)
+static phy_interface_t __maybe_unused tsec_get_interface(struct tsec_private *priv)
 {
 	struct tsec __iomem *regs = priv->regs;
 	u32 ecntrl;