platform/x86: dell-laptop: Fix backlight detection

Fix return code check for "max brightness" ACPI call.

The Dell laptop ACPI video brightness control is not present on dell
laptops anymore, but was present in older kernel versions.

The code that checks the return value is incorrect since the SMM
refactoring.

The old code was:
  if (buffer->output[0] == 0)

Which was changed to:
  ret = dell_send_request(...)
  if (ret)

However, dell_send_request() will return 0 if buffer->output[0] == 0,
so we must change the check to:
  if (ret == 0)

This issue was found on a Dell M4800 laptop, and the fix tested on it
as well.

Fixes: 549b493 ("dell-smbios: Introduce dispatcher for SMM calls")
Signed-off-by: Damien Thébault <[email protected]>
Tested-by: Damien Thébault <[email protected]>
Reviewed-by: Pali Rohár <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Signed-off-by: Andy Shevchenko <[email protected]>

drivers/platform/x86/dell-laptop.c

@@ -2185,7 +2185,7 @@ static int __init dell_init(void)
 		dell_fill_request(&buffer, token->location, 0, 0, 0);
 		ret = dell_send_request(&buffer,
 					CLASS_TOKEN_READ, SELECT_TOKEN_AC);
-		if (ret)
+		if (ret == 0)
 			max_intensity = buffer.output[3];
 	}