mei: expose device state in sysfs

Expose mei device state to user-space through sysfs.
This gives indication to applications that driver is in transition,
usefully mostly to detect link reset state.

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>