powerpc/pseries/vio: Remove stray #ifdef CONFIG_PPC_PSERIES

vio.c is in platforms/pseries, which is only built if PPC_PSERIES=y.
In other words, this ifdef is pointless.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200130063153.19915-1-oohall@gmail.com