staging: most: clean up configuration strings

This patch adds the strings 'rx', 'tx' and 'isoc' to the list of accepted
identifiers when setting up a channel configuration. To keep consistency it
removes the prefix "dir_" from strings returned by the attributes
set_direction and available_directions and it removes the suffix "_avp"
from the string "isoc_avp" returned by the attributes set_datatype and
available_datatypes.

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