HID: hid-debug: clean up snprintf() checks in hid_resolv_usage()

The snprintf() limits are complicated and slightly wrong when it does:

	max(0, HID_DEBUG_BUFSIZE - len - 1)

The "- 1" should not be there.  It means we can't use the last
byte of the buffer.  If we change the first snprintf() to scnprintf()
then we can remove the max().

At the start of the function the strlen(buf) is going always going to
be < HID_DEBUG_BUFSIZE so that is safe.  If it were > HID_DEBUG_BUFSIZE
then that would result in a WARN().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>