docs: spi.h: removed dev doxygen comment from struct spi_config
docs: spi.h: removed dev doxygen comment from struct spi_config

Removed a doxygen comment for a structure variable that has been
removed from spi_config.

Signed-off-by: AJ Palmer <502f8b00176fc044a53dff00593a5b9a8ec2b69f@hotmail.com>

zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr