device_add() may fail, and all callers are supposed to check the
return value, but one new user in lightnvm doesn't:
drivers/lightnvm/sysfs.c: In function 'nvm_sysfs_register_dev':
drivers/lightnvm/sysfs.c:184:2: error: ignoring return value of 'device_add',
declared with attribute warn_unused_result [-Werror=unused-result]
This changes the caller to propagate any error codes, which avoids
the warning.