USB: Add new USB LPM helpers

Use new helpers to make LPM enabling/disabling more clear.

This is a preparation to subsequent patch.

Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: stable <stable@vger.kernel.org> # after much soaking
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>