serial-core: disable power managment for serial tx

1% of the time where the getty never appears on ttyS1 even after our
timeout of 1000s.

When this happens we've added code to login to the ttyS0 getty and run
debug commands. We've been able to confirm the getty is running and the
init system doesn't matter (happens with sysvinit and systemd). The
most interesting debug I've seen is this:

root@qemux86-64:~# cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:418 rx:43 RTS|CTS|DTR|DSR|CD
1: uart:16550A port:000002F8 irq:3 tx:249 rx:0 RTS|CTS|DTR|DSR|CD
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3
root@qemux86-64:~# echo helloA > /dev/ttyS1
root@qemux86-64:~# echo helloB > /dev/ttyS0
helloB
root@qemux86-64:~# cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:803 rx:121 RTS|CTS|DTR|DSR|CD
1: uart:16550A port:000002F8 irq:3 tx:281 rx:0 RTS|CTS|DTR|DSR|CD
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3

This is being run after the getty didn't appear for 60s on ttyS1 so
we've logged into ttyS0 and run these commands. We've seen that if it
doesn't appear after 60s, it won't appear after 1000s either.

The tx:249 is interesting as it should be tx:273, 273 being the number
of bytes our successful serial getty prompt has. Once we echo something
to the port (8 bytes), tx: jumps to 281, so it suddenly found our
missing login prompt. This is confirmed with the data appearing on the
port after the echo.

I did try disabling the autosuspend code in the commit above but it
made no difference. What does seem to help is changing the conditional
the patch adds around start_tx() back to being under the original
conditions. This is relatively harmless as it will just stop_tx() again
if the xmit buffer is empty and this is a one off operation at probe
time.
The small overhead is much preferred to randomly failing tests.

Discussions with upstream are being attempted:
https://lore.kernel.org/linux-serial/c85ab969826989c27402711155ec086fd81574fb.camel@linuxfoundation.org/T/#t



Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>