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

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: 65131cd52b9e ("c2port: add c2port support for Eurotech Duramar 2150")
Signed-off-by: Dan Carpenter <[email protected]>
Acked-by: Rodolfo Giometti <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>