mei: use only one buffer in callback

mei: use only one buffer in callback

The callback structure is used exclusively for reading or writing
therefore there is no reason to hold both response and request buffers
in the callback structure

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