regulator: twl: Make sure we have access to powerbus before trying to write to it

According to the TRM, we need to enable i2c access to powerbus before
writing to it. Also, a new write to powerbus should not be attempted if
there is a pending transfer. The current code does not implement that
functionality and while there are no known problems caused by that, it is
better to follow what TRM says.

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>