tg3: Refactor tg3_close()

by introducing tg3_stop() that does the opposite of tg3_start().  This
function will be useful when adding the support for changing the numbe
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>