staging: comedi: comedi_fc: export cfc_bytes_per_scan()

This helper function is a bit long to be inlined. Move it to the
source file and export it.

The mite driver is the only external user right now. Update the
Kconfig to select COMEDI_FC when COMEDI_MITE is enabled.

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>