checkpatch: add ignore for network block comment style checking

When use checkpatch.pl to check network related patch, it will report
--->8---
WARNING: networking block comments don't use an empty /* line,
use /* Comment...
---<8---

So, add --ignore NETWORKING_BLOCK_COMMENT_STYLE into .checkpatch.conf
This will help to keep all driver include network related driver use
the same comment style

Signed-off-by: Bo Shen <voice.shen@atmel.com>