platform/x86: dell-smbios-wmi: make function dell_smbios_wmi_call static

The function dell_smbios_wmi_call is local to the source and does not
need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'dell_smbios_wmi_call' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>