mei: bus: make a client pointer always available

Change life time of the client pointer, allocate it upon client device
creation and free it upon device destruction, instead of upon
connection and disconnection.
This helps to eliminate racy NULL checks in the bus code.

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