firewire: core: use inline helper functions to serialize phy config packet

This commit uses the added helper functions to obsolete the existing
implementation for phy configuration packet.

Link: https://lore.kernel.org/r/20240606235133.231543-3-o-takashi@sakamocchi.jp


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