greybus: sdio: rework of event handler

Between the time connection with module is up and the host is added,
we can receive events (card inserted/removed, write protection
switch), so until the setup is complete we queue the events received
and handle them after.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>