net: lwm2m: use BIT macro instead of LWM2M_OP_BIT

Now that the LWM2M_OP_* bits have been renumbered, we no longer need
a custom BIT() macro for the LwM2M code.  Let's remove it and use
BIT() instead.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>