Revert "ramips: add support for kernel 5.10"

This reverts commit b4aad29a1d.

This was accidentally folded into a single commit. Remove it and
apply it properly again.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ramips/patches-5.10/840-serial-add-ugly-custom-baud-rate-hack.patch

@@ -1,22 +0,0 @@
-From a7eb46e0ea4a11e4dfb56ab129bf816d1059a6c5 Mon Sep 17 00:00:00 2001
-From: John Crispin <blogic@openwrt.org>
-Date: Mon, 7 Dec 2015 17:31:08 +0100
-Subject: [PATCH 51/53] serial: add ugly custom baud rate hack
-
-Signed-off-by: John Crispin <blogic@openwrt.org>
----
- drivers/tty/serial/serial_core.c |    3 +++
- 1 file changed, 3 insertions(+)
-
---- a/drivers/tty/serial/serial_core.c
-+++ b/drivers/tty/serial/serial_core.c
-@@ -418,6 +418,9 @@ uart_get_baud_rate(struct uart_port *por
- 		break;
- 	}
- 
-+	if (tty_termios_baud_rate(termios) == 2500000)
-+		return 250000;
-+
- 	for (try = 0; try < 2; try++) {
- 		baud = tty_termios_baud_rate(termios);
-