USB: serial: continue to read on errors

Make sure to try to resubmit the read urb on errors.

Currently a recoverable error would lead to reduced throughput as only
one urb will be used until the port is closed and reopened (or
resumed or unthrottled).

Also upgrade error messages from debug to error log level.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>