[ARM] pxa: fix interrupts number calculation when CONFIG_PXA_HAVE_ISA_IRQS=y

Commit d2c37068 ([ARM] pxa:
initialize default interrupt priority and use ICHP for IRQ handling)
broke ISA interrupt support on pxa27x/3xx.

In such a case, PXA_IRQ(0) != 0, and the IRQ number computed from
ICHP must be offset by PXA_IRQ(0).

Tested on an Arcom Zeus (pxa270), with both CONFIG_PXA_HAVE_ISA_IRQS
enabled and disabled.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Tested-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>