stm32/spi: Allow SPI peripheral state to persist across a soft reset.

The SPI sub-system is independent from the uPy state (eg the heap) and so
can safely persist across a soft reset.  And this is actually necessary for
drivers that rely on SPI and that also need to persist across soft reset
(eg external SPI flash memory).

ports/stm32/main.c

@@ -457,6 +457,7 @@ int main(void) {
 #if MICROPY_HW_HAS_SWITCH
     switch_init0();
 #endif
+    spi_init0();
 
 #if defined(USE_DEVICE_MODE)
     // default to internal flash being the usb medium
@@ -556,7 +557,6 @@ soft_reset:
     i2c_init0();
     #endif
 
-    spi_init0();
     pyb_usb_init0();
 
     // Initialise the local flash filesystem.

ports/stm32/spi.c

@@ -135,29 +135,24 @@ STATIC const pyb_spi_obj_t pyb_spi_obj[] = {
 };
 
 void spi_init0(void) {
-    // reset the SPI handles
+    // Initialise the SPI handles.
+    // The structs live on the BSS so all other fields will be zero after a reset.
     #if defined(MICROPY_HW_SPI1_SCK)
-    memset(&SPIHandle1, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle1.Instance = SPI1;
     #endif
     #if defined(MICROPY_HW_SPI2_SCK)
-    memset(&SPIHandle2, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle2.Instance = SPI2;
     #endif
     #if defined(MICROPY_HW_SPI3_SCK)
-    memset(&SPIHandle3, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle3.Instance = SPI3;
     #endif
     #if defined(MICROPY_HW_SPI4_SCK)
-    memset(&SPIHandle4, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle4.Instance = SPI4;
     #endif
     #if defined(MICROPY_HW_SPI5_SCK)
-    memset(&SPIHandle5, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle5.Instance = SPI5;
     #endif
     #if defined(MICROPY_HW_SPI6_SCK)
-    memset(&SPIHandle6, 0, sizeof(SPI_HandleTypeDef));
     SPIHandle6.Instance = SPI6;
     #endif
 }