usb: musb: gadget: kill duplicate code in musb_gadget_queue()

musb_gadget_queue() checks for '!req->buf' condition twice:
in the second case the code is both duplicated and unreachable
as the first check returns early.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>