coccinelle: ifnullfree: various cleanups

Adjust tests to compare against NULL, to match cases that explicitly make
that comparison.

Remove removal and re-addition of freeing functions.

Add position variable on usb_free_urb in the non-patch case.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>