lightnvm: propagate device_add() error code

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.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 38c9e260b9f9 ("lightnvm: expose device geometry through sysfs")
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>