staging: comedi: jr3_pci: remove next_time_max member

The `next_time_max` member of `struct jr3_pci_subdev_private` is
assigned to, but never read.  Remove it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>