serial: imx: initialize the local variable

The slave_config is not initialized, so some of its fields may
be set with random data which may causes the failure in the following
dmaengine_prep_slave_sg().

This patch fixes this issue.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>