wifi: wilc1000: simplify remain on channel support

For 'struct wilc_remain_ch', drop set but otherwise unused 'duration'
field and adjust 'expired' callback assuming that the only data passed
to it is 'struct wilc_vif *', thus making 'wilc_remain_on_channel()'
a bit simpler as well. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231026141016.71407-1-dmantipov@yandex.ru