spi/spi-bcm2708: respect per-transfer SPI clock speed_hz value

The bcm2708 SPI driver's bcm2708_process_transfer() was ignoring the
per-transfer speed_hz value even when it was provided (it always just
used the spi device's max_speed_hz value).  Now, per-transfer speed_hz
values are respected.

Also added debug print to bcm2708_setup_state() to help keep an eye on
the configured SPI parameters.

Signed-off-by: Kamal Mostafa <kamal@whence.com>