cfg80211: Reset beacon interval when stop AP fails

When SSR happens at WLAN driver, the cfg80211 stop AP can fail at driver.
Make sure that the beacon interval is reset, even when this API fails.

CRs-fixed: 1078172
Change-Id: I459f55ce5f4bc44c4d0e20170bd50a83c2d609b4
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>