regmap: rbtree: Fix wrong register marked as in-cache when creating new node

When regcache_rbtree_write() creates a new rbtree_node it was passing the
wrong bit number to regcache_rbtree_set_register(). The bit number is the
offset __in number of registers__, but in the case of creating a new block
regcache_rbtree_write() was not dividing by the address stride to get the
number of registers.

Fix this by dividing by map->reg_stride.
Compare with regcache_rbtree_read() where the bit is checked.

This bug meant that the wrong register was marked as present. The register
that was written to the cache could not be read from the cache because it
was not marked as cached. But a nearby register could be marked as having
a cached value even if it was never written to the cache.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: 3f4ff561

 ("regmap: rbtree: Make cache_present bitmap per node")
Link: https://lore.kernel.org/r/20230922153711.28103-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>