mei: kill not used BAR0 length and base variables

1. mem_base and mem_length are not used so we can delete
them
2. add kdoc for mem_addr member of mei_device

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