serial: imx: Disable irqs before requesting them

Disable interrupts before requesting them in order to fix a kernel oops
after lauching a kernel via kexec.

Tested on a imx6sl-evk board.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>