ethernet/atheros/alx: add Killer E2400 device ID

This patch adds the PCI device ID (0xe0a1) and alx_pci_tbl entry for the
Killer E2400 Ethernet controller, modeled after the Killer E2200
controller support (0xe091) already present in the alx driver.

This patch was originally authored by Ben Pope, but it got held up by
issues in the commit message, so I'm resubmitting it on his behalf.

I've extensively used a kernel with this patch on a System76 serw9
laptop and am quite confident it works well (at least on the hardware I
have available for testing).

Note that as a favor to System76, Ubuntu has been carrying this as a
sauce patch in their 4.2 based Wily kernel, which presumably has given
it real-world testing on other E2400 equipped hardware (I don't know of
any Ubuntu kernel bugs filed about it):

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1498633



Signed-off-by: Jason Gerard DeRose <jason@system76.com>
Signed-off-by: Ben Pope <benpope81@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>