net: Mac80211: Remove silly timespec dance

Converting time from one format to another seems to give coders a warm
and fuzzy feeling.

Use the proper interfaces.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John W. Linville <linville@tuxdriver.com>
[fix compile error]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>