MIPS: perf: Remove incorrect odd/even counter handling for I6400

All performance counters on I6400 (odd and even) are capable of counting
any of the available events, so drop current logic of using the extra
bit to determine which counter to use.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Fixes: 4e88a862 ("MIPS: Add cases for CPU_I6400")
Fixes: fd716fca

 ("MIPS: perf: Fix I6400 event numbers")
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15991/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>