staging: comedi: ni_at_a2150: use comedi_bytes_per_sample()

For aesthetics, use the helper function to get the byte size of each sample
instead of using sizeof() the first buffer element.

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>