tty: serial: msm: Only configure MND registers on hw that has it

The registers that msm_serial_set_mnd_regs() writes only exist on
the non-uartdm hardware, so let's return early here if this
function is called on uartdm hardware. This also prevents us from
messing up the uartclk variable if the uartclk rate happens to be
19.2 or 4.8 MHz.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>