mt76: round up length on mt76_wr_copy

When beacon length is not a multiple of 4, the beacon could be sent with
the last 1-3 bytes corrupted. The skb data is guaranteed to have enough
room for reading beyond the end, because it is always followed by
skb_shared_info, so rounding up is safe.
All other callers of mt76_wr_copy have multiple-of-4 length already.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>