staging: most: Eliminate usage of symbolic permissions

Fix checkpatch warnings regarding the use of symbolic permissions.

Where the MOST_CHANNEL_ATTR macro is used, convert to octal
permissions over symbolic.

Where _ATTR is used directly, replace with _ATTR_RW/_ATTR_WO and
update the show/store function names appropriately.

Signed-off-by: Jason Litzinger <jlitzingerdev@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>