AT91: at91sam9261ek board: remove warnings related to use of SPI or SD/MMC

The sd/mmc data structure is not used if SPI is selected. The configuration
of PIO on the board prevent from using both interfaces at the same time
(board dependent).
Remove the warnings at compilation time adding a preprocessor condition.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>