rt2x00: Initialize AMPDU_BA_WINSIZE register

Since we're not using the AMPDU_BA_WINSIZE register to force the BlockAck
window size (we specify it in every TXWI) we should initialize it to 0.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>