usb: dwc3: Allocate and free TRB buffers from ep ops

Move TRB buffer allocation under GSI_EP_OP_PREPARE_TRBS
ep operation. Similarly free TRB buffers under
GSI_EP_OP_FREE_TRBS ep operation. This simplifies to
make decision if TRB and buffer allocation needs to come
from cached or non-cached memory region.

Change-Id: Ife4203060ea8092c16a9d2cd92016fa7f6afb563
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>