net: dsa: mt7530: refactor MT7530_PMEEECR_P()

The MT7530_PMEEECR_P() register is on MT7530, MT7531, and the switch on the
MT7988 SoC. Rename the definition for them to MT753X_PMEEECR_P(). Use the
FIELD_PREP and FIELD_GET macros. Rename GET_LPI_THRESH() and
SET_LPI_THRESH() to LPI_THRESH_GET() and LPI_THRESH_SET().

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