regmap: Try cached read before checking if a hardware read is possible

For some register format types we do not provide a parse_val so we can not do a
hardware read. But a cached read is still possible, so try to read from the
cache first, before checking whether a hardware read is possible. Otherwise the
cache becomes pretty useless for these register types.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>