wil6210: remove reference to preset_chandef

The field preset_chandef of wireless_dev must not be accessed
by the driver because it is private to cfg80211. Store the
monitor channel locally in wil6210_priv instead.

Signed-off-by: Lior David <qca_liord@qca.qualcomm.com>
Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>