Scan all descriptors when checking for fastboot

For Linux, the USB code was only looking at the first
descriptor when searching for the fastboot interface
which caused some devices to not be found.

Also clarify some code by using the actual USB structures
instead of void or char pointers.

Change-Id: I9e4871c4d477ac10ba75bb17a955f176809af289
Signed-off-by: Patrick Tjin <pattjin@google.com>