iscsi_ibft: use virt_to_phys instead of isa_virt_to_bus

As far as I can tell IBFT is a firmware table and has nothing to do with
the good old ISA bus.  And even if it the two would be the same on x86
anyway.  So remove the isa_virt_to_bus call in preparation of eventually
removing that API entirely.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>