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 <ard@linaro.org>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>