drivers: modem: gsm: Rename `r` to `ret`
drivers: modem: gsm: Rename `r` to `ret`

Rename the variable `r` to `ret` for better consistency
with the rest of the driver.

Signed-off-by: Yong Cong Sin <d1951b00420cc51c2da597170bb83d89fa46ae3b@gmail.com>

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