firmware: dmi_scan: Fix dmi_len type

firmware: dmi_scan: Fix dmi_len type

According to SMBIOSv3 specification the length of DMI table can be
up to 32bits wide. So use appropriate type to avoid overflow.

It's obvious that dmi_num theoretically can be more than u16 also,
so it's can be changed to u32 or at least it's better to use int
instead of u16, but on that moment I cannot imagine dmi structure
count more than 65535 and it can require changing type of vars that
work with it. So I didn't correct it.

Acked-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Ivan Khoronzhuk <[email protected]>
Cc: <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>