mei: rename variable names 'file_object' to fp

The driver uses three names file, fp, and file_object for
struct file type. To improve code clarity and adjust to my taste
rename file_object to more common and shorter fp.

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>