net: dsa: mt7530: refactor MT7530_PMCR_P()

The MT7530_PMCR_P() registers are on MT7530, MT7531, and the switch on the
MT7988 SoC. Rename the definition for them to MT753X_PMCR_P(). Bit 15 is
for MT7530 only. Add MT7530 prefix to the definition for bit 15.

Use GENMASK and FIELD_PREP for PMCR_IFG_XMIT().

Rename PMCR_TX_EN and PMCR_RX_EN to PMCR_MAC_TX_EN and PMCR_MAC_TX_EN to
follow the naming on the "MT7621 Giga Switch Programming Guide v0.3",
"MT7531 Reference Manual for Development Board v1.0", and "MT7988A Wi-Fi 7
Generation Router Platform: Datasheet (Open Version) v0.1" documents.

These documents show that PMCR_RX_FC_EN is at bit 5. Correct this along
with renaming it to PMCR_FORCE_RX_FC_EN, and the same for PMCR_TX_FC_EN.

Remove PMCR_SPEED_MASK which doesn't have a use.

Rename the force mode definitions for MT7531 to FORCE_MODE. Add MASK at the
end for the mask that includes all force mode definitions.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>