firewire: core: provide isoc header buffer size outside card driver

firewire: core: provide isoc header buffer size outside card driver

For single-channel isochronous contexts, the header storage size is
hard-coded to PAGE_SIZE. which is inconvenient for protocol
implementations requiring more space.

This commit refactors the code to obtain the header storage size outside
the 1394 OHCI driver.

Link: https://lore.kernel.org/r/20260117142823.440811-8-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>