RESCALE: Correct sign checks

Moving between TOSA version 0.70 and 0.80, some
of the checks between input and output type were
omitted.

Signed-off-by: Dominic Symes <dominic.symes@arm.com>
Change-Id: I8e547f18ec1c36e0e9e3f8db04e58b0f643c781c