staging: mmal-vchiq: Reset buffers_with_vpu on port_enable

Should we go through the timeout failure case with port_disable
not returning all buffers for whatever reason, the
buffers_with_vpu counter gets left at a non-zero value, which
will cause reference counting issues should the instance be
reused.

Reset the count when the port is enabled again, but before
any buffers have been sent to the VPU.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>