ARM: PCI: versatile: fix PCI interrupt setup

This is at odds with the documentation in the file; it says pin 1 on
slots 24,25,26,27 map to IRQs 27,28,29,30, but the function will always
be entered with slot=0 due to the lack of swizzle function.  Fix this
function to behave as the comments say, and use the standard PCI
swizzle.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>