staging: dgnc: Update the TODO file

It turned out that the TODO file contained the invalid information as
some of the work has already been done. This commit updates it with the
current status of what is left to be done.
Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/TODO

-* remove kzalloc casts
 * checkpatch fixes
-* sparse fixes
-* fix use of sizeof(). Example replace sizeof(struct board_t) 
-  with sizeof(*brd) and remove sizeof(char)
-* change name of board_t to dgnc_board
 * split two assignments into the two assignments on two lines;
   don't use two equals signs
 * remove unecessary comments