Switch SD card size message to power-of-10 units

[fhunleth]

Currently, you get this:

Use 58.27 GiB memory card found at /dev/sdc? [Yn] 

This is for a MicroSD card that is labeled "64 GB". Since it seems like SSDs and SDCards are all labeled in power-of-10 units and pretty much every other program besides fwup uses power-of-10 units for these devices, change that message to something like this:

Use 62.57 GB memory card found at /dev/sdc? [Yn]