net: airoha: npu: Add wlan_{send,get}_msg NPU callbacks

net: airoha: npu: Add wlan_{send,get}_msg NPU callbacks

Introduce wlan_send_msg() and wlan_get_msg() NPU wlan callbacks used
by the wlan driver (MT76) to initialize NPU module registers in order to
offload wireless-wired traffic.
This is a preliminary patch to enable wlan flowtable offload for EN7581
SoC with MT76 driver.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20250811-airoha-en7581-wlan-offlaod-v7-3-58823603bb4e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>