tile PCI RC: reduce driver's vmalloc space usage

We can take advantage of the fact that bit 29 is hard-wired
to zero in register TRIO_TILE_PIO_REGION_SETUP_CFG_ADDR.
This is handy since at the moment we only allocate one 4GB
region for vmalloc, and with this change we can allocate
four or more TRIO MACs without using up all the vmalloc space.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>