mt76: usb: use max packet length for m76u_copy

For transferring data over USB the optimal size is endpoint maxpacket.
For my hardware maxpaket for control endpoint is 64 bytes and changing
to this value from 128 bytes further shorten TBTT work time from
3ms to 1ms.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>