optee: smc_abi.c: fix wrong pointer passed to IS_ERR/PTR_ERR()

In optee_smc_do_call_with_arg() there is a code path when the argument
struct for RPC is passed appended to the primary argument struct. When
the address of the RPC struct is retrieved there's an invalid check for
success. It should be 'rpc_arg' pass to IS_ERR/PTR_ERR().

Fixes: ed8faf6c ("optee: add OPTEE_SMC_CALL_WITH_RPC_ARG and OPTEE_SMC_CALL_WITH_REGD_ARG")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
[jw: added background to the problem]
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>