roms: minor fixes and cleanups.

Changes:
  - Drop extra file argument from rom_add_file().
  - Drop fw_dir check in do_info_roms, we allways have a dir name.
  - code style fixes.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
