net: tilegx driver: avoid compiler warning

The "id" variable was being incremented in common code, but only
initialized and used in IPv4 code.  We move the increment to the IPv4
code too, and then legitimately use the uninitialized_var() macro to
avoid the gcc 4.6 warning that 'id' may be used uninitialized.
Note that gcc 4.7 does not warn.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>