mac80211: Remove TSF atomic requirement from the documentation

The atomic requirement for the TSF callbacks
is outdated. get_tsf() is only called by
ieee80211_rx_bss_info() which is indirectly
called by the work queue ieee80211_sta_work().
In the same context are called several other
non-atomic functions, too.
And the atomic requirement causes problems
for drivers of USB wifi cards.

Signed-off-by: Alina Friedrichsen <x-alina@gmx.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>