mailbox: check for bit set before polling

Before polling we just need to see if the TXDONE_BY_POLL bit
is set in txdone_method. There may be another bit (method)
specified as well, like TXDONE_BY_ACK.

Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/mailbox.c

···
 exit:
 	spin_unlock_irqrestore(&chan->lock, flags);
 
-	if (!err && chan->txdone_method == TXDONE_BY_POLL)
+	if (!err && (chan->txdone_method & TXDONE_BY_POLL))
 		poll_txdone((unsigned long)chan->mbox);
 }