serial: mxs-auart: fix the wrong setting order

After set the AUART_CTRL0_CLKGATE, the UART will gate all the clocks off.
So the following line will not take effect.
       ................................................................
 	writel(AUART_INTR_RXIEN | AUART_INTR_RTIEN | AUART_INTR_CTSMIEN,
 			u->membase + AUART_INTR_CLR);
       ................................................................

To fix this issue, the patch moves this gate-off line to
the end of setting registers.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>