net: mvpp2: fix a txq_done race condition

When no Tx IRQ is available, the txq_done() routine (called from
tx_done()) shouldn't be called from the polling function, as in such
case it is already called in the Tx path thanks to an hrtimer. This
mostly occurred when using PPv2.1, as the engine then do not have Tx
IRQs.

Fixes: edc660fa

 ("net: mvpp2: replace TX coalescing interrupts with hrtimer")
Reported-by: Stefan Chulski <stefanc@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>