drm/bridge: parade-ps8640: Make sure we drop the AUX mutex in the error case

After commit 26db46bc ("drm/bridge: parade-ps8640: Ensure bridge
is suspended in .post_disable()"), if we hit the error case in
ps8640_aux_transfer() then we return without dropping the mutex. Fix
this oversight.

Fixes: 26db46bc

 ("drm/bridge: parade-ps8640: Ensure bridge is suspended in .post_disable()")
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117103502.1.Ib726a0184913925efc7e99c4d4fc801982e1bc24@changeid