target/ppc: Remove type argument from check_prot

We can now use MMU_INST_FETCH from access_type for this.
Use prot_for_access_type to simplify everything.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210518201146.794854-8-richard.henderson@linaro.org>
[dwg: Remove a stray trailing whitespace]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>