Revert "usb: gadget: rndis: Fix re-binding f_rndis"

This reverts commit 7f141c58.

This patch should not have been applied in the first place. The USB
string id index is limited to 255.  Function drivers are supposed
to allocate string id for the first time their bind is called.  There
is no API to release string id index.  If string id is reset in unbind,
we will run out of string id after multiple bind/unbind.

CRs-Fixed: 425501
Change-Id: Ia82a528f9e0af192063970a9227c9c739ed77f48
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>