mei: amthif: use service function to flush amthif queue

Replace open coded loop with an existing service function:
mei_io_list_flush

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>