Replace UB code by a legitimate pointer access

[samueltardieu]

This removes a compilation error due to the invalid-reference-casting check.

[adamgreig]

In this case the error is legit and the code has UB. It should be updated to at least use &mut self.spi.dr as *mut FrameSize to avoid casting a shared reference to a mutable pointer. Better yet is to use addr_of!() as was done here: stm32-rs/stm32f0xx-hal#170

[samueltardieu]

In this case the error is legit and the code has UB. It should be updated to at least use &mut self.spi.dr as *mut FrameSize to avoid casting a shared reference to a mutable pointer. Better yet is to use addr_of!() as was done here: stm32-rs/stm32f0xx-hal#170

addr_of!() is the way to go, &mut is not possible here as DerefMut is not implemented.