wifi: rtlwifi: fix error codes in rtl_debugfs_set_write_h2c()

If the copy_from_user() fails or the user gives invalid date then the
correct thing to do is to return a negative error code.  (Currently it
returns success).

I made a copy additional related cleanups:
1) There is no need to check "buffer" for NULL.  That's handled by
copy_from_user().
2) The "h2c_len" variable cannot be negative because it is unsigned
and because sscanf() does not return negative error codes.

Fixes: 610247f4 ("rtlwifi: Improve debugging by using debugfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/YoOLnDkHgVltyXK7@kili

drivers/net/wireless/realtek/rtlwifi/debug.c

@@ -329,8 +329,8 @@ static ssize_t rtl_debugfs_set_write_h2c(struct file *filp,
 
 	tmp_len = (count > sizeof(tmp) - 1 ? sizeof(tmp) - 1 : count);
 
-	if (!buffer || copy_from_user(tmp, buffer, tmp_len))
-		return count;
+	if (copy_from_user(tmp, buffer, tmp_len))
+		return -EFAULT;
 
 	tmp[tmp_len] = '\0';
 
@@ -340,8 +340,8 @@ static ssize_t rtl_debugfs_set_write_h2c(struct file *filp,
 			 &h2c_data[4], &h2c_data[5],
 			 &h2c_data[6], &h2c_data[7]);
 
-	if (h2c_len <= 0)
-		return count;
+	if (h2c_len == 0)
+		return -EINVAL;
 
 	for (i = 0; i < h2c_len; i++)
 		h2c_data_packed[i] = (u8)h2c_data[i];