Add check for valid ctx pointer and then only dereference ctx to
configure supp_nowait flag.

Fixes: 42bf4152 ("tee: add supp_nowait flag in tee_context struct")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>