[rilmodem] Minor phonebook code cleanup

Signed-off-by: Tommi Kenakkala <tommi.kenakkala@oss.tieto.com>
[rilmodem] Updated devinfo comments

Signed-off-by: Tommi Kenakkala <tommi.kenakkala@oss.tieto.com>
Merge pull request #245 from tkenakka/master

Minor code cleanup