net: dsa: mt7530: simplify core operations

The core_rmw() function calls core_read_mmd_indirect() to read the
requested register, and then calls core_write_mmd_indirect() to write the
requested value to the register. Because Clause 22 is used to access Clause
45 registers, some operations on core_write_mmd_indirect() are
unnecessarily run. Get rid of core_read_mmd_indirect() and
core_write_mmd_indirect(), and run only the necessary operations on
core_write() and core_rmw().

Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>