src: fix clang15 compiler warnings

Below warnings are fixed.

1- A function declaration without a prototype is deprecated in all
versions of C [-Werror,-Wstrict-prototypes]

2- error: variable set but not used [-Werror,-Wunused-but-set-variable]

Signed-off-by: Erhan Kurubas <erhan.kurubas@espressif.com>
Change-Id: I1cf14b8e5e3e732ebc9cacc4b1cb9009276a8ea9
Reviewed-on: https://review.openocd.org/c/openocd/+/7569


Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins