probe_irq_mask - scan a bitmap of interrupt lines
unsigned int probe_irq_mask (unsigned long val);
val mask of interrupts to consider
Scan the ISA bus interrupt lines and return a bitmap of active interrupts. The interrupt probe logic state is then returned to its previous value.
we need to scan all the irq’s even though we will only return ISA irq numbers - just so that we reset them all to a known state.