net: dsa: sja1105: fix buffer overflow in sja1105_setup_devlink_regions()

If an error occurs in dsa_devlink_region_create(), then 'priv->regions'
array will be accessed by negative index '-1'.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rustam Subkhankulov <subkhankulov@ispras.ru>
Fixes: bf425b82

 ("net: dsa: sja1105: expose static config as devlink region")
Link: https://lore.kernel.org/r/20220817003845.389644-1-subkhankulov@ispras.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>