usb: f_gsi: Use gadget->dev.parent->parent instead of gadget->dev.parent

Currently TRB buffer is allocated using gadget->dev.parent node
(e.g a600000.dwc3) instead of device node (e.g a600000.ssusb)
which is used to create IOMMU based context bank. Hence use
gadget->dev.parent->parent to allocate buffer to avoid unmapped
SMMU page fault while enabling USB SMMU stage S1 functionality.

Change-Id: I6bf9510c7bf6d1daf62e117e2470ed9dc6f7e038
Signed-off-by: Mayank Rana <mrana@codeaurora.org>