drivers/sdcard_spi: fix uint64_t cast location

When compiling `tests/drivers_sdcard_spi` with `avr-gcc: avr-gcc (GCC) 6.4.0`
it detected this error

    RIOT/drivers/sdcard_spi/sdcard_spi.c:1012:72:
    error: result of '512 << 10' requires 21 bits to represent, but 'int' only has 16 bits [-Werror=shift-overflow=]
    return (card->csd.v2.C_SIZE + 1) * (uint64_t)(SD_HC_BLOCK_SIZE << 10);