firmware: dmi_scan: Clarify dmi_save_extended_devices

Get rid of the arbitrary 5-byte pointer offset, it served no purpose
and made it harder to match the code with the SMBIOS specification.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Jordan Hargrave <jordan_hargrave@dell.com>
Cc: Narendra K <narendra_k@dell.com>