drivers/misc/c2port/c2port-duramar2150.c: checking for NULL instead of IS_ERR()

c2port_device_register() never returns NULL, it uses error pointers.

Link: http://lkml.kernel.org/r/20170412083321.GC3250@mwanda
Fixes: 65131cd5

 ("c2port: add c2port support for Eurotech Duramar 2150")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>