mei: move amthif specific code from mei_write to mei_amthif_write

For sake of amthif consolidation move amthif specific code from
mei_write to mei_amthif_write

The original mei_amthif_write to mei_amthif_send_cmd
as this function deals with sending single command
while mei_amthif_write is interface function called from
the main driver which in turns calls mei_amthif_send_cmd

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