powerpc/BSR: fix device_create() return value check

Use IS_ERR() instead of comparing to NULL.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Acked-by: Sonny Rao <sonnyrao@us.ibm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>