fsi: master-ast-cf: Fix memory leak

In case memory resources for *fw* were allocated, release them
before return.

Addresses-Coverity-ID: 1472044 ("Resource leak")
Fixes: 6a794a27 ("fsi: master-ast-cf: Add new FSI master using Aspeed ColdFire")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>