firewire: ohci: Use tasklet_disable_in_atomic() where required

tasklet_disable() is invoked in several places. Some of them are in atomic
context which prevents a conversion of tasklet_disable() to a sleepable
function.

The atomic callchains are:

 ar_context_tasklet()
   ohci_cancel_packet()
     tasklet_disable()

 ...
   ohci_flush_iso_completions()
     tasklet_disable()

The invocation of tasklet_disable() from at_context_flush() is always in
preemptible context.

Use tasklet_disable_in_atomic() for the two invocations in
ohci_cancel_packet() and ohci_flush_iso_completions().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309084242.616379058@linutronix.de