mei: cancel driver workers only after client devices were removed

In process of client devices removal from the bus there still
might be communication between a driver and the mei device
hence we need to cancel supporting workers only after all
the client devices were removed.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>