ath9k_htc: make ath9k_htc_tx_aggr_oper() static

This fixes this sparse complaint:

  CHECK   drivers/net/wireless/ath/ath9k/htc_drv_main.c
drivers/net/wireless/ath/ath9k/htc_drv_main.c:441:5:
	warning: symbol 'ath9k_htc_tx_aggr_oper'
		 was not declared. Should it be static?

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>