media: selftests: ir: fix ir_loopback test failure

The decoder is called rc-mm, not rcmm. This was renamed late in the cycle
so this bug crept in.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>