net: wwan: iosm: Fix tainted pointer delete is case of region creation fail

In case of region creation fail in ipc_devlink_create_region(), previously
created regions delete process starts from tainted pointer which actually
holds error code value.
Fix this bug by decreasing region index before delete.

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

Fixes: 4dcd183f

 ("net: wwan: iosm: devlink registration")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Acked-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240604082500.20769-1-amishin@t-argos.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>