hwspinlock: propagate errno when registering single lock

hwspin_lock_register_single() always returns 0 despite checking the
result from radix_tree_insert(). Propagate the errno to make sanity
checks in callers of this function actually meaningful.

Fixes: 300bab9770e2 ("hwspinlock/core: register a bank of hwspinlocks in a single API call")
Link: https://sashiko.dev/#/patchset/20260319105947.6237-1-wsa%2Brenesas%40sang-engineering.com # review of patch 14
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20260512084856.30497-2-wsa+renesas@sang-engineering.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>