BL31: Fix warning about BL32 init function

BL31: Fix warning about BL32 init function

The expected value for failure is 0, so the warning only has to be shown
in that case. This is the way the TSPD has done it since it was
introduced, and the way SPM and OP-TEE do it.

Trusty wrongly returns 0 on success.

In the case of TLK, the return value of tlkd_init() is passed from the
secure world in register X1 when calling the SMC TLK_ENTRY_DONE.

Change-Id: I39106d67631ee57f109619f8830bf4b9d96155e6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>