usb: gadget: f_qdss: Allocate one string ID for all instances

In case multi instance of driver exist in same composition, id
of the string gets overridden due to multiple bind() calls. This
results in STALL on GET_DESCRIPTOR (string type) on interface
string descriptor index which got overridden. Fix this issue by
assigning one string ID to all interface string desc index of
the driver.

Change-Id: If61455e9786175a1dd435320bfb54f2949e4ffa4
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>