tg3: Refactor tg3_open()

by introducing tg3_start() that handles all initialization steps from
IRQ allocation.  This function will be needed when adding support for
changing the number of rx and tx rings.

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