firewire: Generalize get_config_rom to get_info.

Repurpose the get_config_rom ioctl to a general get_info ioctl.
This ioctl is now used for version negotiation, and optionally
returns the config rom, and the current bus info.

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>