i.MX: Fix UART driver to work with unitialized "chardev" device

The "chardev" property initialization might have failed (for example because
there are not enough chardevs provided by QEMU).

The serial device emulator needs to be able to work with an uninitialized
(NULL) chardev device pointer.

This patch adds some missing tests on the chr pointer value before
using it.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1438342461-18967-1-git-send-email-jcd@tribudubois.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
