staging: comedi: addi_apci_3120: use comedi_bytes_to_samples()

Use the helper function to calculate the number of samples in the DMA
buffer. For aesthetics, tidy up the local variables.

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>