regmap: Tidy up regmap_raw_read chunking code

Raw reads may need to be split into small chunks if max_raw_read is
set.  Tidy up the code implementing this, the new code is slightly
clearer, slightly shorter and slightly more efficient.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>