usb: gadget: f_ecm: Setup netdevice with name 'ecm0'

Currently we are registering net device with the name 'usb0' from
f_ecm. But the netmanager daemon expects the ECM netdevice name
to be 'ecm0' for proper configurations.
Hence change the netdevice name to ecm0.

Change-Id: Iebcdf0ed33dcc0e26e99edc50a33ba6dcb9ea5c5
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>