target/m68k: Fix build Werror with gcc 8.0.1

target/m68k: Fix build Werror with gcc 8.0.1

Fedora 28 ships with the released gcc 8.

The Werror stems from the compiler finding a path through the second
switch via a missing default case in which src1 is uninitialized, and
not being able to prove that the missing default case is unreachable
due to the first switch.

Simplify the second switch to merge default with OS_LONG,
which returns directly.  This removes the unreachable path.

Cc: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-id: 20180508185520.23757-1-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>