regmap: rbtree: Don't bother checking for noop updates

If we're updating a value in place it's more work to read the value and
compare the value with what we're about to set than it is to just write
the value into the cache; there are no further operations after writing
in the code even though there's an early return here.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>