Char: isicom, fix close bug

port is dereferenced even if it is NULL.  Dereference it _after_ the
check if (!port)...  Thanks Eric <ef87@yahoo.com> for reporting this.

This fixes

        http://bugzilla.kernel.org/show_bug.cgi?id=7527



Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>