metag: perf: fix core internal / perf channel mux

The value written to the PERF_ICOREx or PERF_CHANx register to select
the performance events for the core internal and perf channel events was
(tmp & 0x0f), but tmp was set to (config & 0xf0) so it would always be
0. Correct it to use config instead of tmp.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>