Global variables must be reset to their default value before a new
dfu_bind is done. Otherwise things wont work and are likely to cause
a system crash due to a use after free: the global dfu_files was still
pointing deallocated structure after unbind.

Signed-off-by: Jules Maselbas <jmaselbas@kalray.eu>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>