powerpc/powernv/sriov: Remove unused but set variable 'phb'

Gcc report warning as follows:

arch/powerpc/platforms/powernv/pci-sriov.c:602:25: warning:
 variable 'phb' set but not used [-Wunused-but-set-variable]
  602 |  struct pnv_phb        *phb;
      |                         ^~~

This variable is not used, so this commit removing it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200727171112.2781-1-weiyongjun1@huawei.com