i8259: PREP: Replace pic_intack_read with pic_read_irq

i8259: PREP: Replace pic_intack_read with pic_read_irq

There is nothing in the i8259 spec that justifies the special
pic_intack_read. At least the Linux PREP kernels configure the PICs
properly so that pic_read_irq returns identical values, and setting
read_reg_select in PIC0 cannot be derived from any special i8259 mode.

So switch ppc_prep to pic_read_irq and drop the now unused PIC code.

CC: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>