rapidio: fix potential NULL pointer dereference

The change fixes improper check for a returned error value by
class_create() function, which on error returns ERR_PTR() value, thus the
original check always results in a dead code on error path.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>