net: dsa: qca8k: convert to regmap read/write API

Convert qca8k to regmap read/write bulk API. The mgmt eth can write up
to 32 bytes of data at times. Currently we use a custom function to do
it but regmap now supports declaration of read/write bulk even without a
bus.

Drop the custom function and rework the regmap function to this new
implementation.

Rework the qca8k_fdb_read/write function to use the new
regmap_bulk_read/write as the old qca8k_bulk_read/write are now dropped.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>