firewire: ohci: use inline functions to operate data of self-ID DMA

The code of 1394 OHCI driver includes hard-coded magic number to operate
data of Self-ID DMA.

This commit replaces them with the inline functions added/tested in the
former commit.

Link: https://lore.kernel.org/r/20240702222034.1378764-5-o-takashi@sakamocchi.jp


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