serial: mvebu-uart: Fix to avoid a potential NULL pointer dereference

of_match_device on failure to find a matching device can return a NULL
pointer. The patch checks for such a scenrio and passes the error upstream.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>