spi: meson-spicc: do not rely on busy flag in pow2 clk ops

Since [1], controller's busy flag isn't set anymore when the
__spi_transfer_message_noqueue() is used instead of the
__spi_pump_transfer_message() logic for spi_sync transfers.

Since the pow2 clock ops were limited to only be available when a
transfer is ongoing (between prepare_transfer_hardware and
unprepare_transfer_hardware callbacks), the only way to track this
down is to check for the controller cur_msg.

[1] ae7d2346 ("spi: Don't use the message queue if possible in spi_sync")

Fixes: 09992025 ("spi: meson-spicc: add local pow2 clock ops to preserve rate between messages")
Fixes: ae7d2346

 ("spi: Don't use the message queue if possible in spi_sync")
Reported-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/r/20220908121803.919943-1-narmstrong@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>