staging: fsl-mc/dpio: Fix the error handling in probe()

First of all devm_memremap() function returns an error pointer, it
doesn't return NULL.  And second we need to set the error code on these
paths.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>