net: ieee802154: introduce consistent MTU definition

Zephyr IEEE 802.15.4 drivers and L2 stack use the same constant names
for different MTU definitions. The intent of this change is to introduce
a consistent MTU definition which can be used everywhere in zephyr to
avoid confusion, bugs and name conflict.

Signed-off-by: Florian Grandel <jerico.dev@gmail.com>