[rilmodem] Null netreg pointer on netreg remove

Netreg driver has a static copy of the netreg pointer.
Missing NULLing may cause problems on corner cases after
netreg is freed (when core atom is flushed).

Signed-off-by: Tommi Kenakkala <tommi.kenakkala@oss.tieto.com>
Merge pull request #233 from tkenakka/netregfix

[rilmodem] Null netreg pointer on netreg remove