STM32.SPI: Useless assignments
STM32.SPI: Useless assignments

This was detected by a tool. The code is a little bit strange with this
As_Half_Word_Pointer conversion, but I think it's OK to remove the
assignments.

Fabien-Chouteau/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library,simonjwright/Ada_Drivers_Library,ellamosi/Ada_BMP_Library,simonjwright/Ada_Drivers_Library,Fabien-Chouteau/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library