drivers: ieee802154: properly announce promisc mode

Most IEEE 802.15.4 drivers do not support promiscuous mode, some do.
There is a dedicated L2 flag to signal this capability to clients.

Unfortunately the IEEE 802.15.4 L2 stack does not announce this flag
even for drivers that correctly expose it in their HW capabilities.
Some clients (notably the OpenThread L2) even uses promiscuous mode
without checking whether the driver actually supports it.

This change lets the vanilla IEEE 802.15.4 L2 check the driver's
HW capabilities to announce promiscuous mode on its 'get_flags()'
interface if supported.

The OpenThread L2 uses a constant (potentially incorrect) response
to 'get_flags()'. Fixing the OpenThread L2 is out of scope of this
change. This change just introduces TODO messages to the OpenThread code
so that the OpenThread team may fix the issue (or delete the TODO if they
deem it irrelevant).

Fixes: #51263

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