spi: spidev: fix a debug message value

spi: spidev: fix a debug message value

The debug message in spidev_message() can show wrong xfer speed.
It happens if the initial (came from DT) and set with ioctl call spidev
speeds are different (spidev->speed_hz != spi->max_speed_hz) and one
sends a message with ioctl call and the field of speed is uninitialized
(u_tmp->speed_hz == 0).

In this case the kernel shows the spi->max_speed_hz value instead of
correct spidev->speed_hz.
...
set the max speed with an ioctl call:
[ 1227.702714] spidev spi0.0: setup mode 0, 32 bits/w, 20000000 Hz max --> 0
(real speed sets to 20000000Hz)
send a message with an ioctl call:
[ 1227.731801] spidev spi0.0:   xfer len 4096 tx 32bits 0 usec 10000000Hz
(debug message shows 10000000Hz that is the original max speed of this
spidev came from DT)
...

Fix the data source for the debug message.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Link: https://lore.kernel.org/r/20200229161841.89144-2-oleksandr.suvorov@toradex.com
Signed-off-by: Mark Brown <broonie@kernel.org>