ipmi: Fix attention handling for system interfaces

If an attention came in while handling a message response, it
could cause the state machine to go into the wrong mode and lock
things up if the state machine wasn't in normal mode.  So if the
state machine is not in normal mode, save the attention flag for
later.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Tony Rex <tony.rex@ericsson.com>
Tested-by: Magnus Johansson E <magnus.e.johansson@ericsson.com>
Cc: Per Fogelström <per.fogelstrom@ericsson.com>