mei: amthif: enable poll for async events

Currently the poll function is bailing early for amthif client and
ignores requests for async events notifications.
Move async event processing before amthif to enable async events
notifications on amthif client.

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>