staging: mei: kill struct hbm_cmd bit fields

Replace bitfield struct hbm_cmd with simple u8 as we
always access the value as whole. This allows us to remove
few ugly type casts
For possible further uses and documentation purposes we add
corresponding bitmask defines

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