regmap: regcache_rbtree_{set,get}_register: Use regcache_{set,get}_val

Use regcache_{set,get}_val in regcache_rbtree_{set,get}_register instead of
re-implementing its functionality.

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