staging: comedi: introduce some sample size manipulation functions

Introduce a few static inline helper functions:

`comedi_bytes_per_sample(s)` is the same as the existing
`bytes_per_sample(s)` and determines the size of a comedi sample in
bytes.  (`bytes_per_sample(s)` will be removed.)

`comedi_sample_shift(s)` determines the log2 of the comedi sample size,
so it can be used in bit-shift operations to multiply or divide by the
sample size.

`comedi_bytes_to_samples(s, nbytes)` converts a number of bytes to a
number of samples (rounding down).

`comedi_samples_to_bytes(s, nsamples)` converts a number of samples to a
number of bytes.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>