drm: aux ->transfer() can return 0, deal with it

Restore the correct behaviour (as in check msg.reply) when aux
->transfer() returns 0. It got removed in
commit 82922da3 ("drm/dp_helper: Retry aux transactions on all errors")

Now I can actually dump the "entire" DPCD on a Dell UP2314Q with
ddrescue. It has some offsets in the DPCD that can't be read
for some resaon, all you get is defers. Previously ddrescue would
just give up at the first unredable offset on account of
read() returning 0 means EOF. Here's the ddrescue log
for the interested:
0x00000000  0x00001400  +
0x00001400  0x00000030  -
0x00001430  0x000001D0  +
0x00001600  0x00000030  -
0x00001630  0x0001F9D0  +
0x00021000  0x00000001  -
0x00021001  0x000DEFFF  +

Cc: Lyude <cpaul@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch
Cc: stable@vger.kernel.org
Fixes: 82922da3 ("drm/dp_helper: Retry aux transactions on all errors")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>