forcedeth: remove overhead

This patch removes unnecessary overhead code. Firstly, there is no nead
to mask off unwanted interrupts as we will be checking against the
irqmask field anyways. Secondly, there has been no value in last few
years from detecting error or unknown interrupts.

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