cfg80211: fix elapsed_jiffies calculation

MAX_JIFFY_OFFSET has no meaning when calculating the
elapsed jiffies, as jiffies run out until ULONG_MAX.

This miscalculation results in erroneous values
in case of a wrap-around.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>