xtensa: iss/network: provide release() callback

[ Upstream commit 8864fb83 ]

Provide release() callback for the platform device embedded into struct
iss_net_private and registered in the iss_net_configure so that
platform_device_unregister could be called for it.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>