dfu: modify an argument type for an address

The range of an addressable pointer can go beyond 'integer'.
So change the argument type to a void pointer.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>