kni: fix build with debian kernel 3.2.57-2

Following debian kernel headers upgrade to 3.2.57, pci capability accessors
have been backported (upstream commit 8c0d3a02c1309eb6112d2e7c8172e8ceb26ecfca,
("PCI: Add accessors for PCI Express Capability", v3.7-rc1)).

It results in the same compilation error as redhat 6.x.
However, there is no clear way to determine we are building on a debian kernel.
So, rather than determine if we are building on a distribution kernel, look at
PCI_EXP_LNKSTA2 that appeared in this upstream commit.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>