nand: Correct NAND erase percentage output

For NAND erase sizes smaller than one NAND erase block, erase
percentage output becomes grater than 100% e.g.

-- cut --
  > nand info
Device 0: NAND 64MiB 1,8V 8-bit, sector size 16 KiB
  > nand erase 0x100000 0x2000
NAND erase: device 0 offset 0x100000, size 0x2000
Erasing at 0x100000 -- 200% complete.
OK
  >
-- cut --

Correct this and give user a warning that more is erased than specified:

-- cut --
  > nand erase 0x100000 0x2000
NAND erase: device 0 offset 0x100000, size 0x2000
Warning: Erase size 0x00002000 smaller than one erase block 0x00004000
           Erasing 0x00004000 instead
Erasing at 0x100000 -- 100% complete.
OK
  >
-- cut --

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>