drm/prime: clarify logic a bit in drm_gem_prime_fd_to_handle

if (!ret) implies that ret == 0, so no need to clear it again. And
explicitly check for ret == 0 to indicate that we're checking an errno
integer.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>