mei: name space for mei device state

1. add MEI_DEV_ prefix for mei device state enums
2. rename mei_state to dev_state
3. add constant to string translation for debug purposes

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>