firewire: fw-sbp2: always enable IRQs before calling command ORB callback

On IOMMU-less noncoherent architectures, orb->callback will memcpy the
whole SCSI command buffer for READ-like SCSI commands.  It is therefore
friendlier to enable IRQs before the call, like before patch "Add
ref-counting for sbp2 orbs".

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Kristian Høgsberg <krh@redhat.com>