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

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: 09b6c112 ("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