ath6kl: Parse beacon interval from userspace

Parse beacon interval from userspace to
firmware. Incase the firmware does not
supports it, just print a warning message
and continue with AP settings.

Cc: Sumathi Mandipati <sumathi@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>