openssl: fix EVP_PKEY_CTX_get_rsa_pss_saltlen() not returning a value

Backport a patch from upstream. Specifically, this fixes signature
validation in trusted-firmware-a with OpenSSL 3.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>