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

Commit 0b50dc4f ("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: 0b50dc4f ("Convert smsc911x to use ACPI as well as DT")
Cc: Jeremy Linton <jeremy.linton@arm.com>
Cc Graeme Gregory <graeme.gregory@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>