i2o: Fix I/O space alignment requirement

When i2o_iop_systab_set() allocates I/O port space, it specifies 1Mb
alignment required.  This seems unlikely, since most platforms have only
64Kb of I/O space total.  I think 4Kb is a more reasonable choice, since
that's the minimum alignment of a PCI-PCI bridge I/O window.

My guess is that this is a copy/paste error from the memory allocation
code, which specifies 1Mb alignment (which is the minimum alignment of a
PCI-PCI bridge memory window).

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