drivers: wifi: eswifi: fix dead code in ipv4 parsing
drivers: wifi: eswifi: fix dead code in ipv4 parsing

unsigned byte value is initialized to -1 and so never satisifies
the while condition, leading to broken ipv4 parsing (dead code).
Fix this by making byte value signed.

Reported by coverity #27326.

Signed-off-by: Loic Poulain <7dbc6498a90ff1ada42fc224430b4f763baa5c5c@linaro.org>

Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr