lte-softmodem: fix the wrongly Nid_cell

Make sure that we fetch the cell id from config
file rather than always fix it to 0.

Signed-off-by: Mhd Zaher Mahfouz <zaher@tmrnd.com.my>
Signed-off-by: Chun-Yeow Yeoh <yeow@tmrnd.com.my>