regmap: Fix regmap_bulk_write for bus writes

The regmap config does not prohibit val_bytes that are not powers of
two. But the current code of regmap_bulk_write for use_single_rw does
limit the possible val_bytes to 1, 2 and 4.

This patch fixes the behaviour to allow bus writes with non-standard
val_bytes sizes.

Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>