tg3: Add flag to disable 1G Half Duplex advertisement

Some link partners have issues if the non-standard 1G half duplex is
advertised. This patch adds support for an nvram setting to disable the
advertisement.

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>