ath10k: remove set but not used variable 'tlv_len'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/wireless/ath/ath10k/wmi-tlv.c: In function 'ath10k_wmi_tlv_op_gen_config_pno_start':
drivers/net/wireless/ath/ath10k/wmi-tlv.c:3455:6: warning:
 variable 'tlv_len' set but not used [-Wunused-but-set-variable]

It never used since inroduction in commit
ce834e28 ("ath10k: support NET_DETECT WoWLAN feature")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>