mei: amthif: drop iamthif_current_cb

iamthif_current_cb was used in request cancel in amthif code.
Now a canceled request is discarded only at the end of the processing
and the variable lost its purpose and can be safely removed.

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>