ath5k: Fix wrong function name in comments

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/ath/ath5k/pcu.c:865: warning: expecting prototype for at5k_hw_stop_rx_pcu(). Prototype was for ath5k_hw_stop_rx_pcu() instead

Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Yang Shen <shenyang39@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210517050141.61488-2-shenyang39@huawei.com