drivers: wifi: Fix TX buffers leak

During interface down in case TX has pending buffers in either TXQ or
Pending_Q then they are not freed instead the Q itself is freed.

Fix by traversing the Q and freeing all members.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>

drivers/wifi/nrfwifi/src/shim.c

@@ -235,6 +235,10 @@ static void *zep_shim_nbuf_alloc(unsigned int size)
 
 static void zep_shim_nbuf_free(void *nbuf)
 {
+	if (!nbuf) {
+		return;
+	}
+
 	k_free(((struct nwb *)nbuf)->priv);
 	k_free(nbuf);
 }