staging: comedi: add comedi_bytes_per_scan()

The "comedi_fc" module contains a few functions useful to Comedi
drivers.  Their functionality is being migrated to the core "comedi"
module and renamed to start with the prefix `comedi_`.  As part of this
migration, move `cfc_bytes_per_scan()` into the core comedi module and
rename it to `comedi_bytes_per_scan()`.  Change the external declaration
of `cfc_bytes_per_scan()` into an inline function that calls
`comedi_bytes_per_scan()`.

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>