powerpc/powernv: Fix sysparam sysfs error handling

When a sysparam query in OPAL returned a negative value (error code),
sysfs would spew out a decent chunk of memory; almost 64K more than
expected. This was traced to a sign/unsigned mix up in the OPAL sysparam
sysfs code at sys_param_show.

The return value of sys_param_show is a ssize_t, calculated using

  return ret ? ret : attr->param_size;

Alan Modra explains:

  "attr->param_size" is an unsigned int, "ret" an int, so the overall
  expression has type unsigned int.  Result is that ret is cast to
  unsigned int before being cast to ssize_t.

Instead of using the ternary operator, set ret to the param_size if an
error is not detected. The same bug exists in the sysfs write callback;
this patch fixes it in the same way.

A note on debugging this next time: on my system gcc will warn about
this if compiled with -Wsign-compare, which is not enabled by -Wall,
only -Wextra.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>