firmware: dmi_scan: Use direct access to static vars

There is no reason to pass static vars to function that can use
only them.

The dmi_table() can use only dmi_len and dmi_num static vars, so use
them directly. In this case we can freely change their type in one
place and slightly decrease redundancy.

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