HSBC Holdings Chairman Mark Tucker's drive to reboot the giant lender needs a reboot.The decision by UniCredit's Jean Pierre Mustier to drop out of consideration for HSBC's top job marks the second high-profile setback in less than a week for Tucker, the first outsider ever hired as the chairman of the 165-year-old institution. The bank's shares have tumbled 6.9% since a Feb. 18 strategy overhaul fell flat and on Monday reached their lowest in more than three years.