mei: drop wr_msg from the mei_dev structure

The control messages are usually small, around 8 bytes, and can be
allocated on the stack.
Using on stack allocation allows us to drop 'wr_msg' a rather large
buffer reserved in the mei_dev structure and relax contention
of this device global buffer.

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>