serial: mvebu-uart: initialize over sampling stack register

The baudrate derivation relies on the state of the programmable over
sampling stack (OSAMP register) being empty, while never initializing
it.

Set all the fields of this register to 0 (except reserved areas) to
ensure a x16 divisor, as assumed by the driver.

The suspend/resume callbacks are untouched because they already
save/restore correctly this register.
Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mvebu-uart.c

@@ -73,6 +73,7 @@
 
 #define UART_OSAMP		0x14
 #define  OSAMP_DEFAULT_DIVISOR	16
+#define  OSAMP_DIVISORS_MASK	0x3F3F3F3F
 
 #define MVEBU_NR_UARTS		2
 
@@ -446,7 +447,7 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
 {
 	struct mvebu_uart *mvuart = to_mvuart(port);
 	unsigned int d_divisor, m_divisor;
-	u32 brdv;
+	u32 brdv, osamp;
 
 	if (IS_ERR(mvuart->clk))
 		return -PTR_ERR(mvuart->clk);
@@ -469,6 +470,10 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud)
 	brdv |= d_divisor;
 	writel(brdv, port->membase + UART_BRDV);
 
+	osamp = readl(port->membase + UART_OSAMP);
+	osamp &= ~OSAMP_DIVISORS_MASK;
+	writel(osamp, port->membase + UART_OSAMP);
+
 	return 0;
 }