ohci: allocate timer only once.

Allocate timer once, at init time, instead of allocating/freeing
it all the time when starting/stopping the bus.  Simplifies the
code, also fixes bugs (memory leak) due to missing checks whenever
the time is already allocated or not.

Cc: Prasad J Pandit <pjp@fedoraproject.org>
Reported-by: Zuozhi Fzz <zuozhi.fzz@alibaba-inc.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
