firewire: core: use mutex to coordinate concurrent calls to flush completions

firewire: core: use mutex to coordinate concurrent calls to flush completions

In current implementation, test_and_set_bit_lock() is used to mediate
concurrent calls of ohci_flush_iso_completions(). However, the ad-hoc
usage of atomic operations is not preferable.

This commit uses mutex_trylock() as the similar operations. The core
function is responsible for the mediation, instead of 1394 OHCI driver.

Link: https://lore.kernel.org/r/20240909140018.65289-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>