firewire: cdev: replace some spin_lock_irqsave by spin_lock_irq

All of these functions are entered with IRQs enabled.
Hence the unconditional spin_unlock_irq can be used.

Function:                  Caller context:
    dequeue_event()            client process, via read(2)
    fill_bus_reset_event()     fw-device.c update worqueue job
    release_client_resource()  client process, via ioctl(2)
    fw_device_op_release()     client process, via close(2)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>