ath10k: work around uninitialized vht_pfr variable

As clang points out, the vht_pfr is assigned to a struct member
without being initialized in one case:

drivers/net/wireless/ath/ath10k/mac.c:7528:7: error: variable 'vht_pfr' is used uninitialized whenever 'if' condition
      is false [-Werror,-Wsometimes-uninitialized]
                if (!ath10k_mac_can_set_bitrate_mask(ar, band, mask,
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath10k/mac.c:7551:20: note: uninitialized use occurs here
                arvif->vht_pfr = vht_pfr;
                                 ^~~~~~~
drivers/net/wireless/ath/ath10k/mac.c:7528:3: note: remove the 'if' if its condition is always true
                if (!ath10k_mac_can_set_bitrate_mask(ar, band, mask,
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath10k/mac.c:7483:12: note: initialize the variable 'vht_pfr' to silence this warning
        u8 vht_pfr;

Add an explicit but probably incorrect initialization here.
I suspect we want a better fix here, but chose this approach to
illustrate the issue.

Fixes: 8b97b055 ("ath10k: fix failure to set multiple fixed rate")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ath/ath10k/mac.c

@@ -7541,6 +7541,8 @@ static int ath10k_mac_op_set_bitrate_mask(struct ieee80211_hw *hw,
 								&vht_nss,
 								true);
 			update_bitrate_mask = false;
+		} else {
+			vht_pfr = 0;
 		}
 
 		mutex_lock(&ar->conf_mutex);