A hardware bug means that reads through the expansion ROM BAR can
return corrupted data if the PEGs are running. This breaks platforms
that re-read the expansion ROM after invoking gPXE code, such as IBM
blade servers.
Halt PEGs during driver shutdown, and unhalt PEGs during driver
startup if we detect that this is not the first startup since
power-on.
Most other Phantom drivers define a register space in terms of a 64M
virtual address space. While this doesn't map in any meaningful way
to the actual addresses used on the latest cards, it makes maintenance
easier if we do the same.
