Remove the global nodemgr_serialize mutex which enclosed most of the
host thread event loop.  This allows for parallelism between several
host adapter cards.

Properly serialize the driver hooks .update(), .suspend(), .resume(),
and .remove() by means of device->sem.  These hooks can be called from
outside the host threads' contexts.

Get() and put() the device.driver when calling its hooks.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>