staging: comedi: comedi_fc: tidy up cfc_bytes_per_scan()

This function returns the total number of bytes for a full scan of
an async command chanlist. The returned value is an unsigned int.

Change all the local variables to unsigned int and use a local
variable for the s->async->cmd.chanlist_len.

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>