tspd:FWU:Fix usage of SMC_RET0

SMC_RET0 should only be used when the SMC code works as a function that
returns void. If the code of the SMC uses SMC_RET1 to return a value to
signify success and doesn't return anything in case of an error (or the
other way around) SMC_RET1 should always be used to return clearly
identifiable values.

This patch fixes two cases in which the code used SMC_RET0 instead of
SMC_RET1.

It also introduces the define SMC_OK to use when an SMC must return a
value to tell that it succeeded, the same way as SMC_UNK is used in case
of failure.

Change-Id: Ie4278b51559e4262aced13bbde4e844023270582
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>