serial: zs: Fix a transmit lockup in console output

Transmit interrupts are disabled and the transmit buffer drained in the
course of console output so that polled transmission is possible. That
however causes a lost transmit interrupt as the TxIP bit in RR3 is only
set on a transmit buffer full-to-empty transition and then iff transmit
interrupts are enabled at the same time. Consequently if console output
disturbs a regular transmission in progress, the TxIP bit is never set
again and the transmission locks up waiting for a transmit interrupt.

Fix the problem by restarting transmission manually rather than waiting
for a transmit interrupt that will never happen.

Signed-off-by: Maciej W. Rozycki <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/zs.c

···
 	if (txint & TxINT_ENAB) {
 		zport->regs[1] |= TxINT_ENAB;
 		write_zsreg(zport, R1, zport->regs[1]);
+
+		/* Resume any transmission as the TxIP bit won't be set.  */
+		if (!zport->tx_stopped)
+			zs_raw_transmit_chars(zport);
 	}
 	spin_unlock_irqrestore(&scc->zlock, flags);
 }