spi: imx: add a check for speed_hz before calculating the clock

spi: imx: add a check for speed_hz before calculating the clock

When some drivers use spi to send data, spi_transfer->speed_hz is
not assigned. If spidev->max_speed_hz is not assigned as well, it
will cause an error in configuring the clock.
Add a check for these two values before configuring the clock. An
error will be returned when they are not assigned.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Link: https://lore.kernel.org/r/20210408103347.244313-2-xiaoning.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>