forcedeth: remove msix + napi

This patch removes support for msix running in conjunction with napi.
There has been reported issues regarding the behaviour of irqmask and
generation of interrupts by the HW when in MSIX mode. When running napi,
the driver is constantly turning off/on the irqmask. For the time being,
I am going to disable it until I can root cause the issue.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>