usb: gadget: Clear string index for RMNET

String index for rmnet/QDSS is not cleared during unbind,
hence whenever rmnet/QDSS interface is used it won’t
request for string index instead use the previously
received index, which may conflict with other interface.

Change-Id: I94f357ae3271f8fc9b746e7bfca59a0487bbc777
Signed-off-by: Chetan C R <quic_cchinnad@quicinc.com>