serial: 8250: Name MSR literals

Add UART_MSR_STATUS_BITS for CD, RI, DSR & CTS. Use names for the
literal.

Don't make the define for combined flags part of UAPI.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221125130509.8482-3-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -1268,9 +1268,9 @@ static void autoconfig(struct uart_8250_port *up)
 	 */
 	if (!(port->flags & UPF_SKIP_TEST)) {
 		serial8250_out_MCR(up, UART_MCR_LOOP | 0x0A);
-		status1 = serial_in(up, UART_MSR) & 0xF0;
+		status1 = serial_in(up, UART_MSR) & UART_MSR_STATUS_BITS;
 		serial8250_out_MCR(up, save_mcr);
-		if (status1 != 0x90) {
+		if (status1 != (UART_MSR_DCD | UART_MSR_CTS)) {
 			spin_unlock_irqrestore(&port->lock, flags);
 			DEBUG_AUTOCONF("LOOP test failed (%02x) ",
 				       status1);

include/linux/serial.h

@@ -28,6 +28,11 @@ static inline bool uart_lsr_tx_empty(u16 lsr)
 	return (lsr & UART_LSR_BOTH_EMPTY) == UART_LSR_BOTH_EMPTY;
 }
 
+#define UART_MSR_STATUS_BITS	(UART_MSR_DCD | \
+				 UART_MSR_RI | \
+				 UART_MSR_DSR | \
+				 UART_MSR_CTS)
+
 /*
  * Counters of the input lines (CTS, DSR, RI, CD) interrupts
  */