Linux: Use ID_DRIVE_FLASH_SD instead of ID_USB_DRIVER to check for SD card

[fmauch]

As mentioned in #12 checking for ID_DRIVE_FLASH_SD helps recognizing the card

On some systems e.g. with external card readers the drive may show up as USB storage device, while with an internal card reader it might show up with the ID_DRIVE_FLASH_SD property.

I've tested this locally with a micro SD card in the internal card reader, an SD card in an external card reader and a USB flash drive which did also get recognized by the ID_USB_DRIVER='usb-storage' check.