kernel: Enable CONFIG_SPI_SPIDEV when selecting kmod-spidev

Without this patch /dev/spidev*.* only appears if CONFIG_SPI_SPIDEV is manually
selected and set to "y" from the kernel config.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 40784