powerpc/pseries/mobility: Set pr_fmt()

The pr_err() callsites in mobility.c already manually include a
"mobility:" prefix, let's make it official for the benefit of messages
to be added later.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190627053044.9238-2-nathanl@linux.ibm.com