firewire: core: non-atomic memory allocation for isochronous event to user client

firewire: core: non-atomic memory allocation for isochronous event to user client

In the former commits, the callback of isochronous context runs on work
process, thus no need to use atomic memory allocation.

This commit replaces GFP_ATOMIC with GCP_KERNEL in the callback for user
client.

Tested-by: Edmund Raile <edmund.raile@protonmail.com>
Link: https://lore.kernel.org/r/20240904125155.461886-5-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>