ath9k: fix extending the rx timestamp with the hardware TSF

AR5416 and all newer chipsets use a 32 bit rx timestamp, so there
is no need to keep the 15 bit timestamp extending logic around.

This patch removes ath9k_hw_extend_tsf (replaced by a call to
ath9k_hw_gettsf64), and reduces the frequency of TSF reads, which
can improve performance in some cases.

This change also has the side effect of making rx timestamps
more accurate.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>