firewire: core: code refactoring to use idr_for_each_entry() macro instead of idr_for_each() function

This commit is a preparation to use xa_for_each() macro. Current
implementation uses idr_for_each() function and has a disadvantage to
replace with the macro. The IDR framework has idr_for_each_entry() macro
for the similar purpose. This commit replace the function with the
macro with minor code refactoring.

Link: https://lore.kernel.org/r/20240812235210.28458-5-o-takashi@sakamocchi.jpSigned-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>