drivers: fix printf formatting in flash drivers

This commit fixes following issues in printf formatting used by flash
drivers:
- cast values of type off_t to long to remove warnings generated when
  compiling with Newlib.
- use 'z' modifier (as in "%zu") to print values of type size_t
- prefix all hex numbers with '0x'

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>