serial: 8250: Adjust misleading LSR related comment

serial8250_rx_chars() has max_count based character limit. If it
triggers, the function returns the old LSR value (and it has never
returned only flags which were not handled). Adjust the comment to
match behavior and warn about which flags can be depended on.

I'd have moved LSR read before LSR read and used serial_lsr_in() also
here but I came across an old discussion about the topic. That
discussion generated commit d22f8f1068 ("serial: 8250: Fix lost rx
state") so I left the code as it is (it works as long as the callers
only use a subset of the LSR flags which holds true today) and changed
the comment instead.

Link: https://www.spinics.net/lists/linux-serial/msg16220.html
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220608095431.18376-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1783,9 +1783,11 @@ void serial8250_read_char(struct uart_8250_port *up, unsigned char lsr)
 EXPORT_SYMBOL_GPL(serial8250_read_char);
 
 /*
- * serial8250_rx_chars: processes according to the passed in LSR
- * value, and returns the remaining LSR bits not handled
- * by this Rx routine.
+ * serial8250_rx_chars - Read characters. The first LSR value must be passed in.
+ *
+ * Returns LSR bits. The caller should rely only on non-Rx related LSR bits
+ * (such as THRE) because the LSR value might come from an already consumed
+ * character.
  */
 unsigned char serial8250_rx_chars(struct uart_8250_port *up, unsigned char lsr)
 {