net: dsa: microchip: dcb: add comments for DSCP related functions

All other functions are commented. Add missing comments to following
functions:
ksz_set_global_dscp_entry()
ksz_port_add_dscp_prio()
ksz_port_del_dscp_prio()

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240510053828.2412516-3-o.rempel@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>