mei: drop mei_io_cb_alloc_buf

mei_io_cb_alloc_buf have a single caller :mei_cl_alloc_cb. After amthif
stopped using it, the code can be integrated into the caller and the
function can be dropped.

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>