regmap: Simplify the initiation of async I/O

Rather than passing a flag around through the entire call stack store it
in the regmap struct and read it when required. This minimises the
visibility of the feature through the API, minimising the code updates
needed to use it more widely.

Signed-off-by: Mark Brown <broonie@linaro.org>

diff --cc drivers/base/regmap/internal.h
Simple merge

diff --cc drivers/base/regmap/regcache.c
Simple merge

diff --cc drivers/base/regmap/regmap.c
Simple merge