media: venus: hfi: Correct session init return error

The hfi_session_init can be called many times and it returns
EINVAL when the session was already initialized. This error code
(EINVAL) is confusing for the callers. Change hfi_session_init to
return EALREADY error code when the session has been already
initialized.

Tested-by: Fritz Koenig <frkoenig@chromium.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>