hal: stm32cube: Fix warning when SPI STM32F0 LL API is compiled

Current implementation of LL_SPI_TransmitData16 on F0 family
generates following warning:
"warning: dereferencing type-punned pointer will break strict-aliasing
rules [-Wstrict-aliasing]"
Besides being forbidden by rule, this cast is not needed, as register is
16 bits wide. Modification has been tested on F0 SoC.
stm32yyxx_ll_spi.h being included in soc.h file, warning is generated
at each compiled object, this commit allows a clean build.
This issue is referenced in ST and tracked under
reference  13359. Code will be updated on upcoming stm32cube updates.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>