i2o: Fix I/O space allocation copy/paste error

When i2o_iop_systab_set() allocates I/O port space, it assigns the base of
the new I/O port region to sb->current_mem_base, not sb->current_io_base.
This looks like a copy/paste error, because we do use current_io_base, but
there's no other place that sets it.

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