staging: comedi: addi_apci_3120: properly disable interrupts in apci3120_cancel()

The APCI3120_DISABLE_ALL_INTERRUPT define is a mask value used to clear bits
in devpriv->b_ModeSelectRegister to disable the interrupt sources. Writing this
value directly sets unintended bits.

Just set the devpriv->b_ModeSelectRegister to '0' and write that to the register.

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>