serial: 8250_omap: refactor mdr1 update

The silicon errata[1] workaround implemented in a follow-on
patch, "serial: 8250_omap: workaround errata on disabling
UART after using DMA", requires MDR1 register programming.

Extract MDR1 register update into helper function,
omap8250_update_mdr1() to help with that.

[1] Advisory 21 in http://www.ti.com/lit/er/sprz408b/sprz408b.pdf



Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>