ath6kl: Retain bg scan period value modified by the user

Added a new member bg_scan_period in struct ath6kl_vif
to retain background scan period value configured via debugfs
entry 'bgscan_interval'. This backup is needed in schedule scan
path while configuring scan parameters.

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