usb: dwc3: debug: fix string position formatting mixup with ret and len

Currently the string formatting is mixing up the offset of ret and
len. Re-work the code to use just len, remove ret and use scnprintf
instead of snprintf and len position accumulation where required.
Remove the -ve return check since scnprintf never returns a failure
-ve size. Also break overly long lines to clean up checkpatch
warnings.

Addresses-Coverity: ("Unused value")
Fixes: 1381a511 ("usb: dwc3: debug: purge usage of strcat")
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>