powerpc/xmon: Fix -Wswitch-unreachable warning in bpt_cmds

powerpc/xmon: Fix -Wswitch-unreachable warning in bpt_cmds

[ Upstream commit 1c4a4a4c8410be4a231a58b23e7a30923ff954ac ]

When building with automatic stack variable initialization, GCC 12
complains about variables defined outside of switch case statements.
Move the variable into the case that uses it, which silences the warning:

arch/powerpc/xmon/xmon.c: In function ‘bpt_cmds’:
arch/powerpc/xmon/xmon.c:1529:13: warning: statement will never be executed [-Wswitch-unreachable]
 1529 |         int mode;
      |             ^~~~

Fixes: 09b6c1129f89 ("powerpc/xmon: Fix compile error with PPC_8xx=y")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YySE6FHiOcbWWR+9@work
Signed-off-by: Sasha Levin <sashal@kernel.org>