ice: Replace ice specific DSCP mapping num with a kernel define

ice: Replace ice specific DSCP mapping num with a kernel define

Replace ice driver specific DSCP mapping number defines
ICE_DSCP_NUM_VAL and IIDC_MAX_DSCP_MAPPING with
an equivalent kernel define DSCP_MAX.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Tatyana Nikolova <tatyana.e.nikolova@intel.com>
Signed-off-by: Dave Ertman <david.m.ertman@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>