firewire: core: obsolete usage of GFP_ATOMIC at building node tree

The flag of GFP_ATOMIC is given to the call of kmalloc when building node
tree, but the call is not atomic context. The call of
fw_core_handle_bus_reset() and fw_core_remove_card() builds the tree,
while they are done in specific workqueue or pci remove callback.

This commit obsolete the usage of GFP_ATOMIC.

Link: https://lore.kernel.org/r/20230604070255.172700-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>