disk/part.c: Fix device enumeration through API

disk/part.c: Fix device enumeration through API

The patch below fixes device enumeration through the U-Boot API.

Device enumeration crashes when the system in question doesn't
have any RAM mapped to address zero (I discovered this on a
BeagleBone board), since the enumeration calls get_dev with a
NULL ifname sometimes which then gets passed down to strncmp().

This fix simply ensures that get_dev returns NULL when invoked
with a NULL ifname.

Signed-off-by: Tim Kientzle <kientzle@freebsd.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>