firmware: dmi_scan: Trim DMI table length before exporting it

The SMBIOS v3 entry points specify a maximum length for the DMI table,
not the exact length. Thus there may be garbage after the end-of-table
marker, which we don't want to export to user-space. Adjust dmi_len
when we find the end-of-table marker, so that only the actual table
payload is exported.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@globallogic.com>