target: esp_xtensa_smp: fix clang report

target: esp_xtensa_smp: fix clang report

The variable 'smp_break' is only set and used in case of smp.
But clang cannot track if 'target->smp' get modified between the
set and the use of 'smp_break', so it consider possible to use
'smp_break' uninitialized.

Initialize 'smp_break' to silent clang.

Change-Id: Ifa25a84fe9ffa25b8b58d7920ec77994c3b7ebfe
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 77287b8d47b4 ("target: add Espressif ESP32 basic support")
Reviewed-on: https://review.openocd.org/c/openocd/+/7050
Reviewed-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Tested-by: jenkins