firewire: core: use mutex instead of spinlock for client isochronous context

firewire: core: use mutex instead of spinlock for client isochronous context

There is a restriction that the userspace client associated to a file
descriptor can hold one isochronous context. The client-level spinning
lock is used to guarantee it, however the lock is also used for
multi-purposes. Additionally, there is no need to use this type of lock,
and the mutex is available, instead.

Link: https://lore.kernel.org/r/20260110013911.19160-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>