net: Increasing the default number of network contexts

The old default 2 is too low especially if TCP is enabled.
So in order not to confuse the application developer,
increase the default number of network contexts to 6.

Change-Id: I263bb4b6f31354a11d921d94aa97214abd85ae24
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>