firewire: ohci: use kcalloc() variant for array allocation

When allocating the list of isochronous context structure, a kzalloc()
variant of managed device API is used. In this case, a kcalloc() variant
is available.

This commit replaces these lines with devm_kcalloc().

Link: https://lore.kernel.org/r/20250908012108.514698-2-o-takashi@sakamocchi.jp


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