drm/i915: error_buffer->ring should be signed

drm/i915: error_buffer->ring should be signed

gcc seems to get uber-anal recently about these things.

Clarification from Dan Carpenter:

"Sorry, I should have said that it's not a gcc warning, it's a smatch
thing.  But also it's not uber-anal.  It's the exact level of anality
which is required to make the == -1 test work.  You can compare
unsigned int and longs to -1 and it works but for smaller types it
doesn't."

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>