RT4318: Fix OSSL_SSIZE_MAX for UEFI build

Commit e634b448

 ("Defines OSSL_SSIZE_MAX") introduced a definition of
OSSL_SSIZE_MAX which broke the UEFI build. Fix that by making UEFI take
the same definition as Ultrix (ssize_t == int).

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>