serial: rda-uart: Don't allow CS5-6

Only CS7 and CS8 are supported but CSIZE is not sanitized after
fallthrough from CS5 or CS6 to CS7.

Set CSIZE correctly so that userspace knows the effective value.
Incorrect CSIZE also results in miscalculation of the frame bits in
tty_get_char_size() or in its predecessor where the roughly the same
code is directly within uart_update_timeout().

Fixes: c10b1332 (tty: serial: Add RDA8810PL UART driver)
Cc: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220519081808.3776-4-ilpo.jarvinen@linux.intel.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/rda-uart.c

@@ -262,6 +262,8 @@ static void rda_uart_set_termios(struct uart_port *port,
 		fallthrough;
 	case CS7:
 		ctrl &= ~RDA_UART_DBITS_8;
+		termios->c_cflag &= ~CSIZE;
+		termios->c_cflag |= CS7;
 		break;
 	default:
 		ctrl |= RDA_UART_DBITS_8;