wifi: mt76: mt7915: fix bounds checking for tx-free-done command

According to the tx-free-done documentation, the DW4 can be repeated,
so have to be more careful about how we test for walking off the
end of the array.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>