usb: gadget: rndis: Fix re-binding f_rndis

f_rndis checks if rndis_string_defs[0].id is null to setup rndis
and allocate string ids when it is bound to the first configuration:

	/* maybe allocate device-global string IDs */
	if (rndis_string_defs[0].id == 0) {

		/* ... and setup RNDIS itself */
		status = rndis_init();
		if (status < 0)
			return status;

rndis_string_defs[0].id must be reset to 0 on unbind for rndis to be
correctly initialized on the next composite_bind.

Signed-off-by: Benoit Goby <benoit@android.com>