usb: gadget: proper skb_reserve usage

Only calls skb_reserve once, after the skb is allocated.
This avoids a situation where the head is constantly
progressing towards the end - thereby causing an skb_panic.

Change-Id: I90375673cd8e2e442a2612ca9b4ca58b7080f206
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>