can: mpc5xxx_can: check of_iomap return before use

of_iomap() can return NULL so that return needs to be checked and NULL
treated as failure. While at it also take care of the missing
of_node_put() in the error path.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: commit afa17a50 ("net/can: add driver for mscan family & mpc52xx_mscan")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>