[eisa] Check for system board presence before probing for slots

[eisa] Check for system board presence before probing for slots

EISA expansion slot I/O port addresses overlap space that may be
assigned to PCI devices, which can lead to register reads and writes
with unwanted side effects during EISA probing.

Reduce the chances of performing EISA probing on PCI devices by
probing EISA slot vendor and product ID registers only if the EISA
system board vendor ID register indicates that the motherboard
supports EISA.

Debugged-by: Václav Ovsík <vaclav.ovsik@gmail.com>
Tested-by: Václav Ovsík <vaclav.ovsik@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>