tg3: Enhance firmware download code to support fragmented firmware

This lays the ground work to download the 57766 fragmented firmware. We
loop until we've written data equal to tp->fw->size minus headers.

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>