tg3: Refactor cpu pause/resume code

The 57766 rxcpu needs to be paused/resumed when we download the firmware just
like we do for existing firmware. Refactor the pause/resume code to be
reusable.

This patch also renames the "offset" argument of tg3_halt_cpu to "cpu_base"
since that's what it really is.

Reviewed-by: Benjamin Li <benli@broadcom.com>
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>