smsc911x: Ignore error return from device_get_phy_mode()

Commit 62ee783b ("smsc911x: Fix crash seen if neither ACPI nor OF is
configured or used") introduces an error check for the return value from
device_get_phy_mode() and bails out if there is an error. Unfortunately,
there are configurations where no phy is configured. Those configurations
now fail.

To fix the problem, accept error returns from device_get_phy_mode(),
and use the return value from device_property_read_u32() to determine
if there is a suitable firmware interface to read the configuration.

Fixes: 62ee783b ("smsc911x: Fix crash seen if neither ACPI nor OF is configured or used")
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>