net: fsl-mc: NULL check dflt_dpio before dereference

net: fsl-mc: NULL check dflt_dpio before dereference

In dpio_exit there is a NULL check for dflt_dpio but it happens after
dpio_dflt has been dereferenced a number of times already. Instead move
the NULL check to first thing in the function. Also assign NULL to
dflt_dpio after free in both dpio_init and dpio_exit.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>