platform/x86: dell-smbios: only run if proper oem string is detected

The proper way to indicate that a system is a 'supported' Dell System
is by the presence of this string in OEM strings.

Allowing the driver to load on non-Dell systems will have undefined
results.

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