driver-core: document ERR_PTR() return values

A number of functions in the driver core return ERR_PTR() values on
error. Document this in the kernel-doc of the functions.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>