dell-smbios: make the SMBIOS buffer static

As dell-laptop has been changed to always retrieve a pointer to the
SMBIOS buffer using dell_smbios_get_buffer(), the SMBIOS buffer can be
marked static.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>