tg3: Set the MAC clock to the fastest speed during boot code load

On the 5717, 5718 and 5719 devices, the bootcode runs slower when any
port doesn't have a link due to clock speed slowing down as part of the
link-aware feature. This leads to the driver timing out waiting for the
bootcode signature.

This patch overrides the clock policy to the highest frequency just before
reset and restores it after the bootcode is up.

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>