usb: pch_udc: unlock on allocation failure

There was an unlock missing on the error path.

Also I did a small cleanup by changing ep->dev->lock for just dev->lock.
They're the same lock, but dev->lock is shorter and that's how it is
used for the spin_unlock_irqrestore() call.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>