serial: imx: ensure UCR3 and UFCR are setup correctly

Commit e61c38d85b73 ("serial: imx: setup DCEDTE early and ensure DCD and
RI irqs to be off") has a flaw: While UCR3 and UFCR were modified using
read-modify-write before it switched to write register values
independent of the previous state. That's a good idea in principle (and
that's why I did it) but needs more care.

This patch reinstates read-modify-write for UFCR and for UCR3 ensures
that RXDMUXSEL and ADNIMP are set for post imx1.

Fixes: e61c38d85b73 ("serial: imx: setup DCEDTE early and ensure DCD and RI irqs to be off")
Signed-off-by: Uwe Kleine-König <[e-mail]>
Acked-by: Mika Penttilä <[e-mail]>
Tested-by: Mika Penttilä <[e-mail]>
Acked-by: Steve Twiss <[e-mail]>
Tested-by: Steve Twiss <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/imx.c

···
 		 * and DCD (when they are outputs) or enables the respective
 		 * irqs. So set this bit early, i.e. before requesting irqs.
 		 */
-		writel(UFCR_DCEDTE, sport->port.membase + UFCR);
+		reg = readl(sport->port.membase + UFCR);
+		if (!(reg & UFCR_DCEDTE))
+			writel(reg | UFCR_DCEDTE, sport->port.membase + UFCR);
 
 		/*
 		 * Disable UCR3_RI and UCR3_DCD irqs. They are also not
···
 		       sport->port.membase + UCR3);
 
 	} else {
-		writel(0, sport->port.membase + UFCR);
+		unsigned long ucr3 = UCR3_DSR;
+
+		reg = readl(sport->port.membase + UFCR);
+		if (reg & UFCR_DCEDTE)
+			writel(reg & ~UFCR_DCEDTE, sport->port.membase + UFCR);
+
+		if (!is_imx1_uart(sport))
+			ucr3 |= IMX21_UCR3_RXDMUXSEL | UCR3_ADNIMP;
+		writel(ucr3, sport->port.membase + UCR3);
 	}
 
 	clk_disable_unprepare(sport->clk_ipg);