staging: tidspbridge: no need for custom NULL

kernel has it's own NULL define, we dont need to introduce our own
custom NULL type!

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>