iscsi_ibft: Fix isa_bus_to_virt not working under ARM

The isa_bus_to_virt is only needed under X86 and in fact the code
that sets the ibft_phys_addr is only compiled under X86.

As such lets just ifdef the code.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Vijayendra Suman <vijayendra.suman@oracle.com>
CC: Maurizio Lombardi <mlombard@redhat.com>
CC: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
v2: Remove the ibft_phys_addr as it is defined in iscsi_ibft.h