USB: unify endpoints' TX and RX buffers

According to the USB standard a USB endpoint can carry data in only one
direction, either from the host computer to the device (OUT endpoint) or
from the device to the host computer (IN endpoint).

So there is no reason to use distinct buffers for trasnmit and receive.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4375 38d2e660-2303-0410-9eaa-f027e97ec537