staging: rt18188eu: Remove useless cast on NULL

Cast on NULL to a pointer type is not required. This
patch removes NULL casted to some pointer type.

The sematic patch used is:
@r@
type T;
@@
- (T *)NULL
+ NULL

Build tested it.

Signed-off-by: Tapasweni Pathak <tapaswenipathak@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>