iwlwifi: mvm: silence unintialized variable warning

"max_amsdu_len" isn't set if kstrtouint() fails.

index 362a546..cb8db23 100644
@@ -1020,6 +1020,8 @@ static ssize_t iwl_dbgfs_max_amsdu_len_write(struct iwl_mvm *mvm,
 	int ret;
 
 	ret = kstrtouint(buf, 0, &max_amsdu_len);
+	if (ret)
+		return ret;
 
 	if (max_amsdu_len > IEEE80211_MAX_MPDU_LEN_VHT_11454)
 		return -EINVAL;

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