forcedeth: realtek phy crossover detection

This patch fixes an issue seen with the realtek 8201 phy. This phy has a
problem with crossover detection and it needs to be disabled. The
problem only arises on certain switches. Therefore, a module parameter
has been added to allow enabling crossover detection if needed. The
default will be set to disabled.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>