regmap: Merge redundant handling in regmap_bulk_write

The handling for the first two cases in regmap_bulk_write is
essentially identical. The first case is just a better implementation of
the second, supporting 8 byte registers and doing the locking manually to
avoid bouncing the lock for each register. Drop some redundant code by
removing the second of these cases and allowing both situations to be
handled by the same code.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>