net: phy: Add phy_interface_is_rgmii helper

RGMII interfaces come in 4 different flavors that the PHY library needs
to care about: regular RGMII (no delays), RGMII with either RX or TX
delay, and both. In order to avoid errors of checking only for one type
of RGMII interface and miss the 3 others, introduce a convenience
function which tests for all values.

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/phy.h b/include/linux/phy.h
index 701c7a3..a26c3f8 100644
--- a/include/linux/phy.h
+++ b/include/linux/phy.h
@@ -678,6 +678,17 @@
 }
 
 /**
+ * phy_interface_is_rgmii - Convenience function for testing if a PHY interface
+ * is RGMII (all variants)
+ * @phydev: the phy_device struct
+ */
+static inline bool phy_interface_is_rgmii(struct phy_device *phydev)
+{
+	return phydev->interface >= PHY_INTERFACE_MODE_RGMII &&
+		phydev->interface <= PHY_INTERFACE_MODE_RGMII_TXID;
+}
+
+/**
  * phy_write_mmd - Convenience function for writing a register
  * on an MMD on a given PHY.
  * @phydev: The phy_device struct