fel: sid: fix stack overflow while reading from SID

When reading from the SID device using the normal memory access method,
we upload our "readl" routine (via fel_readl_n()), which expects a number
of *words* to read. However length is given in *bytes*, so we read four
times as much, and overflow our key buffer, clobbering the return address.
This is typically fatal:
===============
$ ./sunxi-fel sid
02c05200:12345678:34567890:76543210
Segmentation fault (core dumped)
$
===============

Fix this by giving the number of (32-bit) words instead. We already
checked that length is a multiple of 4, so we can just divide.

Signed-off-by: Andre Przywara <osp@andrep.de>

fel_lib.c

@@ -623,7 +623,7 @@ int fel_read_sid(feldev_handle *dev, uint32_t *result,
  else
  /* Read SID directly from memory */
  fel_readl_n(dev, soc->sid_base + soc->sid_offset + offset,
- result, length);
+ result, length / 4);
  return 0;
 }