staging: comedi: serial2002: Fix bare unsigned styling issue

checkpatch.pl warns about a bare unsigned. Add type int to make
explicit and suit the coding style.

Signed-off-by Chris Cesare <chris.cesare@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>