drivers/staging/comedi/comedi_fops.c: add missing vfree

aux_free is freed on all other exits from the function.  By removing the
return, we can benefit from the vfree already at the end of the function.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>