Make ar71xx_spi attach the next free unit of spibus and not only spibus0.
Make ar71xx_spi attach the next free unit of spibus and not only spibus0.

Approved by:	adrian (mentor)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase