boards: stm32f3_disco: LD7 GPIO port is E
boards: stm32f3_disco: LD7 GPIO port is E

Fix LED7 gpio port to E instead of D.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

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