Update bcm2708.c

Hi, 

If I set 'CONFIG_SPI', I can't use my own SPI driver. The SPI bus was already in use (used by spidev module). However if I do not set the 'CONFIG_SPI' driver, I can't use the built-in SPI driver neither. It turns out that the spidev module is built even though the CONFIG_SPI_SPIDEV kernel parameter is not set. I think this is a bug which is resolved by inserting an '#ifdef CONFIG_SPI_SPIDEV' in the spi_board_info struct.

Greetings,
Brecht.

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spidev.c should not be compiled if CONFIG_SPI_SPIDEV is not set. If CONFIG_SPI_SPIDEV is not set, the bcm2708_spi_device variable should not contain the spidev module in order to be able to use another spi driver.