platform/x86: dell-wmi: don't check length returned

This is intended to be variable and provided by the platform.
Some platforms this year will be adopting a 32k WMI buffer, so don't
complain when encountering those platforms or any other future changes.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Reviewed-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>