USB: ffs-test: fix header path

When compiling this program the functionfs.h header cannot be found, producing:
ffs-test.c:40: fatal error: linux/usb/functionfs.h: No such file or directory

This patch also fixes the following warning:
ffs-test.c:453: warning: format ‘%4d’ expects type ‘int’, but argument 3 has type ‘size_t’

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>