tg3: Add new FW_TSO flag

tg3 used the fw_needed member loosely as a synonym for firmware TSO. Now
that the 57766 needs firmware download support, fw_needed can no longer be
used like this. This patch creates a new FW_TSO flag and changes the
code to use it.

Also rearrange all the TSO flags together in the enum.

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>