regmap: No multi_write support if bus->write does not exist

There is no multi_write support available if we cannot use raw_write.
This is the case if bus->write is not implemented.

This patch adds a condition that we need bus and bus->write so that
can_multi_write is true.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>