[PATCH] cm4000_cs: fix return value check

The return value of class_create() need to be checked with IS_ERR().
And register_chrdev() returns errno on failure.
This patch includes these fixes for cm4000_cs and cm4040_cs.

Signed-off-by: Akinbou Mita <akinobu.mita@gmail.com>
Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>