usb: musb: unmap reqs in musb_gadget_queue()'s error case

If the descriptor is missing the reqeust is never unmapped. This patch
changes this and renames the cleanup label to unlock since there is no
cleanup done. The cleanup would revert the allocation of ressource (i.e.
this dma mapping) but it does not, it simply unlocks and returns.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>