staging: comedi: addi_apci_3xxx: remove sanity checks in i_APCI3XXX_InsnReadAnalogInput()

The comedi core validates the insn->chanspec channel and range before
calling the subdevice (*insn_read) function.

Remove the unnecessary sanity checks.

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>