When I change the setting of CONFIG_EISA, everything rebuilds.  This is
because EISA_bus is declared in <asm/processor.h> which is implicitly
included by just about everything.  This is a silly place to declare it,
so this patch moves it to include/linux/eisa.h.

While I'm at it, I also move the variable definition to
drivers/eisa/eisa-bus.c.  The rest of this patch is fixing up the fallout
from having to include <linux/eisa.h> if you use EISA_bus.