cfg80211: fix DFS channel recovery timeout

The timeout was not properly converted from msecs
to jiffies. As a result channel transition to
NL80211_DFS_USABLE was delayed depending on
CONFIG_HZ configuration, e.g. HZ=100 would delay
the NOP from 30 minutes to 300 minutes.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>