usb: gadget: Fix HW enqueue of SPS transfer

As part of kernel 3.4 merge, this driver was modified such
that TRB of size 0 points to DMA_ADDR_INVALID. This causes
infinite SPS transfer to fail after a single transfer.
The fix changes the TRB to point to 0.

Change-Id: I772631f79582db69dccc7aa1801e69695505b958
Signed-off-by: Amit Blay <ablay@codeaurora.org>