treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/drivers/tty/serial/xilinx_uartps.c b/drivers/tty/serial/xilinx_uartps.c
index 2833f14..a9b1ee2 100644
--- a/drivers/tty/serial/xilinx_uartps.c
+++ b/drivers/tty/serial/xilinx_uartps.c
@@ -544,7 +544,7 @@ static int cdns_uart_clk_notifier_cb(struct notifier_block *nb,
 
 		cdns_uart->baud = cdns_uart_set_baud_rate(cdns_uart->port,
 				cdns_uart->baud);
-		/* fall through */
+		fallthrough;
 	case ABORT_RATE_CHANGE:
 		if (!locked)
 			spin_lock_irqsave(&cdns_uart->port->lock, flags);