usb: gadget: Allow u_qc_ether to support multiple functions

u_qc_ether was originally planned to support only single function
driver instance, either f_qc_ecm or f_qc_rndis.
Android gadget can support composite devices with multiple
configurations, for example dual configuration of qc_rndis & qc_ecm
functions.

This change fixes u_qc_ether to support multiple functions.

CRs-Fixed:  408702
Change-Id: I62fa4ee77a349af7bcf8fa08719d20aa941881d9
Signed-off-by: Amit Blay <ablay@codeaurora.org>