[IA64] incorrect return from ia64_pci_legacy_write()

The function ia64_pci_legacy_write() returns 0 for everything
except errors.  This return value gets sent back to the user from
pci_write_legacy_io(), making it look like every write fails.  The trivial
patch below copies the behavior of the SGI sn machvec and does what
would be expected from something implementing a write() function.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>