stmhal: Set the FatFs partition number when initialising VFS object.

stmhal has MULTI_PARTITION enabled for FatFs and so these values need to be
initialised.

stmhal/sdcard.c

@@ -447,6 +447,7 @@ void sdcard_init_vfs(fs_user_mount_t *vfs) {
     vfs->base.type = &mp_fat_vfs_type;
     vfs->flags |= FSUSER_NATIVE | FSUSER_HAVE_IOCTL;
     vfs->fatfs.drv = vfs;
+    vfs->fatfs.part = 0; // autodetect partition
     vfs->readblocks[0] = (mp_obj_t)&pyb_sdcard_readblocks_obj;
     vfs->readblocks[1] = (mp_obj_t)&pyb_sdcard_obj;
     vfs->readblocks[2] = (mp_obj_t)sdcard_read_blocks; // native version

stmhal/storage.c

@@ -507,6 +507,7 @@ void pyb_flash_init_vfs(fs_user_mount_t *vfs) {
     vfs->base.type = &mp_fat_vfs_type;
     vfs->flags |= FSUSER_NATIVE | FSUSER_HAVE_IOCTL;
     vfs->fatfs.drv = vfs;
+    vfs->fatfs.part = 1; // flash filesystem lives on first partition
     vfs->readblocks[0] = (mp_obj_t)&pyb_flash_readblocks_obj;
     vfs->readblocks[1] = (mp_obj_t)&pyb_flash_obj;
     vfs->readblocks[2] = (mp_obj_t)storage_read_blocks; // native version