usb: gadget: android: Reuse host mac address for ECM

If user is not specifying host mac address to f_ecm
then driver is picking random mac addresses after
every composition switch. This may add additional
delay on host PCs for ip address assignment if
mac address to IP address mapping is kept. This also
helps to use same string IDs for ECM as one of the
string ID is used to pass host mac address to HOST PC.

Change-Id: I9d3c7a232a806e4184c920cc26665c6a0badc5a8
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>