usb: gadget: f_ecm: fix host mac address for multiple instances

In case there are multiple ecm instances, either for multiple
otg controllers, or multiple virtual links using libcomposite,
each instance needs to have its own host mac address string
value for iMACAddress.

Update the source array (ecm_string_defs), every time before
usb_gstrings_attach(). Without that, all links wrongly were
getting the same, last allocated, host mac address, rather
than the correct one, as requested via configfs.

Signed-off-by: Lukasz Nowak <lukasz.nowak@exablue.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>