regcache: Preserve cache synchronization errors in regcache_sync()

regcache_sync() currently stores the return value from both cache
synchronization and selector register rewriting in the same variable.
As a result, a successful selector register rewrite can overwrite an
earlier cache synchronization error, causing regcache_sync() to return
success even though synchronization failed.

Track the two operations with separate return variables and preserve the
cache synchronization error. Errors from rewriting selector registers are
returned only if cache synchronization completed successfully.

Signed-off-by: bui duc phuc <phucduc.bui@gmail.com>
Link: https://patch.msgid.link/20260713050312.38729-2-phucduc.bui@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>