fastboot: Add retry to USB read call.

If the USB connection to the device is reset but is still there
the code should retry to re-connect the device and continue.

This is a short term fix for a bootloader issue.
We should revisit and look for a better solution.

Signed-off-by: Dan Murphy <D.Murphy@motorola.com>
Signed-off-by: Mike Lockwood <lockwood@android.com>