i8259: Fix poll command

i8259: Fix poll command

This was probably never used so far: According to the spec, polling
means ack'ing the pending IRQ and setting its corresponding bit in isr.
Moreover, we have to signal a pending IRQ via bit 7 of the returned
value, and we must not return a spurious IRQ if none is pending.

This implements the poll command without the help of pic_poll_read which
is left untouched as pic_intack_read is still using it.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>