serial: 8250: Use frame time to determine timeout

Rather than using a hard-coded per-character Tx-timeout of 10ms,
use the frame time to determine a timeout value. The value is
doubled to ensure that a timeout is only hit during unexpected
circumstances.

Since the frame time may not be available during early printing,
the previous 10ms value is kept as a fallback.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250107212702.169493-3-john.ogness@linutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>