mac80211: Deinline drv_get/set/reset_tsf()

With this .config: http://busybox.net/~vda/kernel_config_ALLYES_Os

,
after deinlining these functions have sizes and callsite counts
as follows:

drv_get_tsf: 634 bytes, 6 calls
drv_set_tsf: 626 bytes, 2 calls
drv_reset_tsf: 617 bytes, 2 calls

Total size reduction is about 4.2 kbytes.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Johannes Berg <johannes.berg@intel.com>
CC: John Linville <linville@tuxdriver.com>
CC: Michal Kazior <michal.kazior@tieto.com>
CC: linux-wireless@vger.kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>