amd-xgbe: use enums for mailbox cmd and sub_cmds

Instead of using hardcoded values, use enumerations for mailbox command
and sub commands.

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>