g-ether-load: fix mac address read

hexdump has a feature to replace repeated characters on output with an *,
use -v to disable this feature.

Thus

hexdump -e '1/1 "%02X" ":"' /proc/device-tree/ocp/ethernet@4a100000/slave@4a100200/mac-address
90:59:AF:5C:*
C5:debian@arm:~$

hexdump -v -e '1/1 "%02X" ":"' /proc/device-tree/ocp/ethernet@4a100000/slave@4a100200/mac-address
90:59:AF:5C:5C:C5:debian@arm:~$

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>