ath6kl: fix return value in ath6kl_wmi_set_pvb_cmd

When building with W=1, we got one warning as belows:
drivers/net/wireless/ath/ath6kl/wmi.c:3509:6: warning: variable ‘ret’
set but not used [-Wunused-but-set-variable]

At the end of ath6kl_wmi_set_pvb_cmd, it is returned by 0 regardless of
return value of ath6kl_wmi_cmd_send.
This patch fixes return value from 0 to ret that has result of
ath6kl_wmi_cmd_send execution.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>