bcm2708 platform data: add 6lowpan SPI module
spi-bcm2708: Check for NULL spi_message->complete callback

The at86rf230 driver does not use a callback for most messages, which would
have resulted in a NULL pointer dereference here. Documentation does not
indicate whether the complete callback is required, but all other drivers
in fact check it, so it's probably best to treat it as optional.

Signed-off-by: Luke Dashjr <luke-jr+git@utopios.org>
Merge branch 'bugfix_rpi_spi_bcm2708' into rpi-6lowpan