regmap: Cache async work structures

Rather than allocating and deallocating the structures used to manage async
transfers each time we do one keep the structures around as long as the
regmap is around. This should provide a small performance improvement.

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

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

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