spi/rockchip: Don't warn if SPI is busy but disabled

The reference manual from Rockchip claims this about the BSF (SPI Busy
Flag):
* 0 - SPI is idle or disabled
* 1 - SPI is actively transferring data

The above doesn't quite appear to be true.  Specifically I found the
busy bit set when SPI was disabled.  Let's change the WARN_ON() so we
only check the busy bit if the controller was enabled.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>