USB: Fix debug output of ark3116

Fix debug output. Previously, it would output "0xFFFFFFB0" on 32-bit
archs (and probably "0xFFFFFFFFFFFFFFB0" on 64-bits), because buf is
taken as signed char, which is promoted to signed int, while %x always
expects an unsigned int.

Signed-off-by: Jan Engelhardt <jengelh@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>