ID is just a some magic number to make fingerprint from one scanner model
incompatible with another scanner model. Get rid of "magic", declare enum
and use it.
https://bugs.freedesktop.org/show_bug.cgi?id=56956
[dsd@gentoo.org: some trivial changes:
fixed some warnings
fixed fp_img memory leak on error
renamed endpoint constants (calling a bulk endpoint 'ctrl' is confusing,
as a control endpoint is something different)
]
