firewire: ohci: omit spinlock IRQ flags where possible

bus_reset_work() is only called from workqueue thread context.

ohci_set_config_rom() and ohci_allocate_iso_context() perform GFP_KERNEL
memory allocations, therefore they must be called with interrupts
enabled.

Hence these functions may disable and enable local IRQs without having
to track IRQ state.

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