SMBIOS: followup: allow now 512 characters for full format string

SMBIOS: followup: allow now 512 characters for full format string

to acutally allow to set the values requested by #2190 we need to
alter the limit a bit, as else the requested values cannot be save.

Just double it to 512, which should be really enough for this, else
go complain to your software vendor for needing such _hacks_ in the
first place..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>