mtd: nand: use ssize_t instead of size_t to prevent infinite loop

When a all 0xFF buffer is passed to drop_ffs, the no-0xFF check loop
will loop forever.
After the fix, If ssize_t i = -1 and size_t l = i + 1, the value of l
will still be 0 as expected.

Signed-off-by: Tao Hou <hotforest@gmail.com>
Cc: Ben Gardiner <bengardiner@nanometrics.ca>
Cc: Scott Wood <scottwood@freescale.com>