dell-laptop: Clear buffer before each SMBIOS call

Make sure that before initializing SMBIOS call, the input buffer does
not contain any garbage (e.g. values from previous SMBIOS call). This
fixes problems with passing undefined/random parameters to SMBIOS
functions.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>