mei: fix client functions names

Use common prefix for function names:

mei_cl_  - for host clients
mei_me_  - for me clients
mei_io_  - for io callback functions

Because mei_cl holds mei_device back pointer
we can also drop the dev argument from the client
functions

add client.h header to export the clients API

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