mei: constify struct file pointer

The struct file file pointer is used as an opaque handle to for a
connected client, for this part the pointer should be immutable and
should be set to count.

Reviewed-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>