usb: gadget: Optimize DMA mapping of UL path SKB buffers

In order to optmize UL path throughput, a pool of SKB buffers is used in
order to avoid re-allocation of them for each request. However, the DMA
mapping of these buffers was still done before queuing them to the HW.
This patch changes it to be done once after the allocation of the SKB
buffer.

Change-Id: I2f84e935fa73e6fa8dcf32e1b46fca6129d8a02c
Signed-off-by: Danny Segal <dsegal@codeaurora.org>