rt2800: set max_psdu to 3 on usb devices

All Ralink USB devices I have, including old ones, work well with
max_psdu = 3 (64kB tx AMPDUs).

Fix indent on the way.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>