platform/x86: dell-smbios: make a function and a pointer static

The function dell_smbios_smm_call and pointer platform_device are
local to the source and do not need to be in global scope, so make
them static.

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

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>