net: shell: Use net management API for MAC address configuration

Fix a bug in setting MAC address using net_if API, the API doesn't do a
memcpy but just stores the pointer and shell was passing stack pointer.

We can use dynamic allocation but freeing the memory for the MAC address
would be trickier, so, use the net management API and let the underlying
drivers figure out the MAC address memory management.

Signed-off-by: Krishna T <krishna.t@nordicsemi.no>