i2o: Refactor i2o_iop_systab_set() PCI space allocation

Refactor the PCI space allocation in i2o_iop_systab_set().  This might
improve readability slightly, but mainly it is to make the next patch
simpler.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>