subsys: console: Fix signed vs unsigned char issues.
subsys: console: Fix signed vs unsigned char issues.

May lead to warnings/errors with pedantic compilers (like LLVM).

Jira: ZEP-2170

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

aceofall/zephyr-iotos,aceofall/zephyr-iotos,aceofall/zephyr-iotos,aceofall/zephyr-iotos,aceofall/zephyr-iotos