usb: gadget: prevent change of Host MAC address of 'usb0' interface

On windows 7 platform, previously allocated ip address is maintained.
However, Host MAC address of 'usb0' interface is changed when the
tethering driver re-enumerated. Thus, the tethering network driver
can't be allocated ip address from dhcp. It causes connection delay
between host and phone for usb tethering.

This patch prevents from changing Host MAC address of 'usb0' interface.
In other words, this patch maintains the Host MAC address allocated when
first tethering driver although the driver is re-enumerated. However,
after reboot, the Host MAC address can be changed.

Change-Id: I43add9925e9d6d90c56cffbd3ed999104448f818