drivers: modem: gsm: Fix DEVICE_DEFINE() data ptr

The commit 5e97d779 ("device: convert DEVICE_INIT to DEVICE_DEFINE
or SYS_DEVICE_DEFINE") did not convert the gsm_ppp.c properly. The
power management parameter was not set to NULL but was pointing to
the data pointer.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>