net: lwm2m: check for read permission on observe

When processing an observe request we fail to check whether a
resource has the read permission set.  Let's check and if it
doesn't return -EPERM.

NOTE: Also do diligence and return -ENOENT when an object field
cannot be found while looking for the permission.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/8286



Signed-off-by: Michael Scott <mike@foundries.io>