smsc911x: Fix crash seen if neither ACPI nor OF is configured or used

Commit 0b50dc4fc971 ("Convert smsc911x to use ACPI as well as DT") makes
the call to smsc911x_probe_config() unconditional, and no longer fails if
there is no device node. device_get_phy_mode() is called unconditionally,
and if there is no phy node configured returns an error code. This error
code is assigned to phy_interface, and interpreted elsewhere in the code
as valid phy mode. This in turn causes qemu to crash when running a
variant of realview_pb_defconfig.

qemu: hardware error: lan9118_read: Bad reg 0x86

Fixes: 0b50dc4fc971 ("Convert smsc911x to use ACPI as well as DT")
Cc: Jeremy Linton <[e-mail]>
Cc Graeme Gregory <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/smsc/smsc911x.c

···
 static int smsc911x_probe_config(struct smsc911x_platform_config *config,
 				 struct device *dev)
 {
+	int phy_interface;
 	u32 width = 0;
 
 	if (!dev)
 		return -ENODEV;
 
-	config->phy_interface = device_get_phy_mode(dev);
+	phy_interface = device_get_phy_mode(dev);
+	if (phy_interface < 0)
+		return phy_interface;
+
+	config->phy_interface = phy_interface;
 
 	device_get_mac_address(dev, config->mac, ETH_ALEN);