mt76: speed up usb bulk copy

Use larger batches for usb copy to speed this operation up. Otherwise it
would be too slow for copying new beacons or broadcast frames over usb.
Assure, that always a multiple of 4 Bytes is copied, as outlined in
850e8f6f "mt76: round up length on mt76_wr_copy" from Felix Fietkau.

Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>