[intelxl] Read PCI bus:dev.fn number from PFFUNC_RID register

[intelxl] Read PCI bus:dev.fn number from PFFUNC_RID register

For the physical function driver, the transmit queue needs to be
configured to be associated with the relevant physical function
number.  This is currently obtained from the bus:dev.fn address of the
underlying PCI device.

In the case of a virtual machine using the physical function via PCI
passthrough, the PCI bus:dev.fn address within the virtual machine is
unrelated to the real physical function number.  Such a function will
typically be presented to the virtual machine as a single-function
device.  The function number extracted from the PCI bus:dev.fn address
will therefore always be zero.

Fix by reading from the Function Requester ID Information Register,
which always returns the real PCI bus:dev.fn address as used by the
physical host.

Signed-off-by: Michael Brown <mcb30@ipxe.org>