firewire: core: replace magic number with macro

In IEEE 1394 specification, the size of bus information block of
configuration ROM is fixed to 5, thus the offset of root directory is 5.
Current implementation to handle device structures has the hard-coded
offset.

This commit replaces the offset with macro.

Link: https://lore.kernel.org/r/20231221134849.603857-3-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>