mt76: usb: fix endian in mt76u_copy

In contrast to mt76_wr() which we use to program registers,
on mt76_wr_copy() we should not change endian of the data.

Fixes: b40b15e1 ("mt76: add usb support to mt76 layer")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>