modem: hl7800: change sleep mode
modem: hl7800: change sleep mode

Sleep mode 0 (driven by DTR) is only recommended for use
as a legacy option.
Sleep mode 1 is recommend by Sierra Wireless.

Signed-off-by: Ryan Erickson <445db09b136432317bdbb8e4308a30651390c2e2@lairdconnect.com>

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