mbed: Fix calling to BlockDevice::size before initialization

These changes fixes a bug that can cause an uninitialized BlockDevice
to be queried for its size. In some cases, this can return unexpected
results (eg: 0). Move setting block device size after its
initialization.
Co-created: @AGlass0fMilk - George Beckstein

Signed-off-by: Artur Tynecki <artur.tynecki@mobica.com>
Signed-off-by: George Beckstein <george.beckstein@gmail.com>