staging: line6: drop MIDI parameter sysfs attrs

Both pod.c and variax.c expose the device's MIDI parameters as sysfs
attrs.  Knowledge of MIDI constants should be in userspace, not in the
driver.  Drop the sysfs attrs and let userspace interpret parameters it
cares about instead.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>