stm32/boards/STM32F769DISC: Use macro instead of const for flash size.

So that the flash size can be changed in just one place.  Also remove the
duplicate cache entry.

ports/stm32/boards/STM32F769DISC/board_init.c

@@ -9,7 +9,6 @@ const mp_spiflash_config_t spiflash_config = {
     .bus_kind = MP_SPIFLASH_BUS_QSPI,
     .bus.u_qspi.data = NULL,
     .bus.u_qspi.proto = &qspi_proto,
-    .cache = NULL,
     .cache = &spi_bdev_cache,
 };
 

ports/stm32/boards/STM32F769DISC/mpconfigboard.h

@@ -41,7 +41,7 @@ extern struct _spi_bdev_t spi_bdev;
 #if !USE_QSPI_XIP
 #define MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE (0)
 #define MICROPY_HW_BDEV_IOCTL(op, arg) ( \
-    (op) == BDEV_IOCTL_NUM_BLOCKS ? (64 * 1024 * 1024 / FLASH_BLOCK_SIZE) : \
+    (op) == BDEV_IOCTL_NUM_BLOCKS ? ((1 << MICROPY_HW_QSPIFLASH_SIZE_BITS_LOG2) / 8 / FLASH_BLOCK_SIZE) : \
     (op) == BDEV_IOCTL_INIT ? spi_bdev_ioctl(&spi_bdev, (op), (uint32_t)&spiflash_config) : \
     spi_bdev_ioctl(&spi_bdev, (op), (arg)) \
 )