disk: stm32: Select `USE_STM32_HAL_SD_EX` on stm32l4

This commit prepares supporting sdmmc on
stm32l4+ devices. When trying to compile the
sdmmc driver there is a compilation error
because `HAL_SDEx_DriveTransceiver_1_8V_Callback`
is not implemented. We solve this by compiling
also `sd_ex` in cube as this function is implemented
there as weak.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>