powerpc/powernv/sriov: Fix use of uninitialised variable

Initialising the value before using it is generally regarded as a good
idea so do that.

Fixes: 4c51f3e1 ("powerpc/powernv/sriov: Make single PE mode a per-BAR setting")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200803075408.132601-1-oohall@gmail.com