staging: most: do not make interface dependent attrs default for all channels

The channel attribute dbr_size is only relevant for the DIM2 interface. so
is the packets_per_xact for USB. This patch cleans up the driver's ABI by
not showing all attributes by default for each channel, but only on those
they belong to.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>