f_gsi: Update USB DPL string descriptor value

Use "DPL Data" as string descriptor's value with USB DPL interface as host
side application only looks for "DPL" string.

Change-Id: I6b6e3b0f65a0fb1135d40c5c955c2b76c43a3fd6
Signed-off-by: Mayank Rana <mrana@codeaurora.org>