serial: 8250: Store to lsr_save_flags after lsr read

Not all LSR register flags are preserved across reads. Therefore, LSR
readers must store the non-preserved bits into lsr_save_flags.

This fix was initially mixed into feature commit f6f5861 ("serial:
8250: Handle UART without interrupt on TEMT using em485"). However,
that feature change had a flaw and it was reverted to make room for
simpler approach providing the same feature. The embedded fix got
reverted with the feature change.

Re-add the lsr_save_flags fix and properly mark it's a fix.

Link: https://lore.kernel.org/all/[email protected]/T/#m1737eef986bd20cf19593e344cebd7b0244945fc
Fixes: e490c91 ("tty: Add software emulated RS485 support for 8250")
Cc: stable <[email protected]>
Acked-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Ilpo Järvinen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/tty/serial/8250/8250_port.c

@@ -1517,6 +1517,8 @@ static inline void __stop_tx(struct uart_8250_port *p)
 		unsigned char lsr = serial_in(p, UART_LSR);
 		u64 stop_delay = 0;
 
+		p->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
+
 		if (!(lsr & UART_LSR_THRE))
 			return;
 		/*