usb-bus: fix no params

After commit 702f3e0fb52c124c07f215426eeadb70a716643f, the params is
nerver NULL. It should check *params instead of params to determine
whether the params is empty.

Signed-off-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
