target/ppc: used ternary operator when registering MAS

target/ppc: used ternary operator when registering MAS

The write calback decision when registering the MAS SPR has been turned
into a ternary operation, rather than an if-then-else block.

This was done because when building without TCG, even though the
compiler will optimize away the pointers to spr_write_generic*, it
doesn't optimize away the decision and assignment to the local pointer,
creating compiler errors. This cleanup looked better than using ifdefs,
so  we decided to with it.

Signed-off-by: Bruno Larsen (billionai) <bruno.larsen@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210525115355.8254-2-bruno.larsen@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>