Staging: comedi: no need for checking vfree call

vfree() does it's own NULL checking,so no need for check before
calling it.

'pages' is local variable argment,so in v2, it is no need assignment
 to NULL.

Signed-off-by: Figo.zhang <figo1802@gmail.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>