staging: comedi: addi_apci_3120: remove devpriv->mode '0xef' magic value

This mask value is the same as APCI3120_DISABLE_TIMER_COUNTER. Use that
instead and remove the "magic" value.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>