staging: comedi: addi_apci_3120: remove unnecessary devpriv->mode masking

The devpriv->mode is set to '0' at the start of apci3120_ai_insn_read() and
apci3120_cyclic_ai(). There is no need to clear any of the bits or do any
of the additional writes of those cleared bits.

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>