staging: comedi: remove use of 'bytes_per_sample()'

This inline helper function has been replaced with comedi_bytes_per_sample().
The same commit (bf33eb4b4f57) introduced a couple other related helper
functions a manipulate the sample size.

Use the new helper functions to remove the use of 'bytes_per_sample()' and
remove it.

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>