dell-smbios: rename clear_buffer() to dell_smbios_clear_buffer()

As clear_buffer() is exported from the module, it has to be renamed to
something less generic, so add a "dell_smbios_" prefix to the function
name.

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>