ath5k: dont use external sleep clock in AP mode

When using the external sleep clock in AP mode, the
TSF increments too quickly, causing beacon interval
to be much lower than it is supposed to be, resulting
in lots of beacon-not-ready interrupts.

This fixes http://bugzilla.kernel.org/show_bug.cgi?id=14802.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>