staging: comedi: make some variables unsigned in comedi_write()

In `comedi_write()`, the `n` and `m` variables are of type `int`.
Change them to `unsigned int` as they are used to measure a positive
number of bytes.  The `count` variable is also of type `int` and holds
the returned number of bytes written.  Change it to type `ssize_t` to
match the function's return type.

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>