Fix passthrough regression

Fix passthrough regression

pt_init() iterates through the PHP slots independantly of
the assignment that occurs inside __insert_to_pci_slot
which is called by register_real_device(). It assumes
that vslots are assigned in order sarting at PHP_SLOT_START.

This was valid before my change, although why it didn't take the
simpler option of just checking what value had been assigned to
pt_dev->dev.devfn in register_real_device() is a mystery to
me [Simon].  Its also a mystery to what valid circumstance could lead
to pt_init() using 0 (a.k.a. unknown?) as the vslot.

My patch made one the assumptions that pt_init() made about slot numbers
invalid. That is, they don't start at PHP_SLOT_START, they start
wherever there is a free device.

A simple solution seems to be to use the value assigned to
pt_dev->dev.devfn in register_real_device().

Signed-off-by: Simon Horman <horms@verge.net.au>