drivers: ieee802154: Make raw mode and l2 mutually exclusive

Activating both, the IEEE 802.15.4 raw mode /and/ the L2 mode is not
possible and in fact leads to a build error.

It should therefore not be possible to activate both options at the same
time. This is achieved by only offering the (rather exotic) raw mode
once the L2-support for the IEEE 802.15.4 has been switched off.

Fixes: #48715

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