mei: extract AMTHI functions into the amthif.c file

Move AMT Host Interface functions into the new amthif.c file.
All functions has now common prefix: mei_amthif_

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