staging: gasket: fix class create bug handling

class_create() never returns NULL, and this driver should never return
PTR_ERR(NULL) anyway.

Reported-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Zhongze Hu <frankhu@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Reviewed-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>