ath9k: use u32 when calculate tsf

Use u32 while ath9k_hw_gettsf32() and
ath9k_hw_gen_timer_start() require u32.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>