staging: line6: drop midi_mask_receive

The midi_mask_receive sysfs attribute selects the MIDI channels on which
to receive messages.  There is no need to do this at the driver level,
instead the MIDI application endpoints decide which channels to use.
Other drivers don't implement midi_mask_receive so applications cannot
make use of this feature.  Therefore we drop it as part of the effort to
clean up the staging driver.

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