firmware: dmi_scan: Optimize dmi_save_extended_devices

Calling dmi_string_nosave isn't cheap, so avoid calling it twice in a
row for the same string.

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