powerpc/vphn: Check for error from hcall_vphn

There is no value in unpacking associativity, if
H_HOME_NODE_ASSOCIATIVITY hcall has returned an error.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200129135301.24739-2-srikar@linux.vnet.ibm.com