linux_spi: Stop messing up the units of SPI speed

'speed' is stored in Hz, so rename the variable to 'speed_hz' to
clarify any potential confusion. Also, when printing the speed after
setting it with an ioctl, convert it to KHz to match the units given
in the message.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>