wifi: mwifiex: cleanup adapter data

Remove unused and set but unused 'dfs_workqueue', 'dfs_work', and
'scan_channels' members of 'struct mwifiex_adapter', adjust users.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230814142101.60308-1-dmantipov@yandex.ru