cxgb4: Account for stid entries properly in case of IPv6

IPv6 uses 2 TIDs with CLIP enabled and 4 TIDs without CLIP.
Currently we are incrementing STIDs in use by 1 for both IPv4 and IPv6 which
is wrong.

Further, driver currently does not have interface to query if CLIP is programmed
for particular IPv6 address. So, in this patch we increment/decrement TIDs in use
by 4 for IPv6 assuming absence of CLIP. Such assumption keeps us on safe side and
we don't end up allocating more stids for IPv6 than actually supported.

Based on original work by Santosh Rastapur <santosh@chelsio.com>

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>