ath6kl: refactor wmi scan command

ATH6KL_FW_CAPABILITY_STA_P2PDEV_DUPLEX was checked in cfg80211.c which is
a bit awkward when adding more callsites to the scan functions. Refactor
the code to wmi.c so that it's transparent to the callers.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>