firewire: core: convert AR-req handler lock from _irqsave to _bh

fw_core_handle_request() is called by the low-level driver in tasklet
context or process context, and fw_core_add/remove_address_handler() is
called by mid- or high-level code in process context.  So convert
address_handler_lock accesses from those which disable local IRQs to
ones which just disable local softIRQs.

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