disk/part.c: 'usb storage' avoiding overflow when output capacity

Before:
    Marvell>> usb storage
      Device 0: Vendor: StoreJet Rev:  Prod:  Transcend
                Type: Hard Disk
                Capacity: 28759.9 MB = 28.0 GB (488397168 x 512)
After:
    Marvell>> usb storage
      Device 0: Vendor: StoreJet Rev:  Prod:  Transcend
                Type: Hard Disk
                Capacity: 238475.1 MB = 232.8 GB (488397168 x 512)

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>