amd-xgbe: define macros for MAC versions and speed select values

amd-xgbe: define macros for MAC versions and speed select values

Define symbolic constants for MAC hardware version numbers and speed
select register values to improve code readability and maintainability.

This replaces magic numbers like 0x30, 0x33, 0x07, 0x06, etc. with
descriptive macro names that indicate their purpose:

MAC versions:
- XGBE_MAC_VER_30: Baseline version supporting Rx adaptation
- XGBE_MAC_VER_33: P100a platform version

Speed select values for MAC_TCR_SS register:
- XGBE_MAC_SS_10G: 10Gbps XGMII mode
- XGBE_MAC_SS_2_5G_GMII: 2.5Gbps GMII mode (older platforms)
- XGBE_MAC_SS_2_5G_XGMII: 2.5Gbps XGMII mode (P100a)
- XGBE_MAC_SS_1G: 1Gbps mode
- XGBE_MAC_SS_100M: 100Mbps mode
- XGBE_MAC_SS_10M: 10Mbps mode

No functional changes.

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Link: https://patch.msgid.link/20260302044634.1388661-1-Raju.Rangoju@amd.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>