Replace custom approach by %*ph specifier to dump small buffers in hex format.

Unfortunately we can't use print_hex_dump_bytes() here since tha gap is
present, though one familiar with the code may change this.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>