tee: remove unnecessary NULL check in tee_shm_alloc()

Smatch complains that "ctx" isn't checked consistently:

    drivers/tee/tee_shm.c:164 tee_shm_alloc()
    warn: variable dereferenced before check 'ctx' (see line 95)

I audited the callers and "ctx" can't be NULL so the check can be
removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>