staging: dgnc: Remove casting the return value which is a void pointer

Casting the return value which is a void pointer is redundant.
The conversion from void pointer to any other pointer type is
guaranteed by the C programming language.

CC: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>