drivers: modem: gsm: If condition should be bool
drivers: modem: gsm: If condition should be bool

Compare these variables properly so that the condition in an if
statement will be boolean.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

Update gsm_ppp.c

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